









| THE LOOSE LEAF LABORATORY MANUAL 


ms THE WILEY TECHNICAL SERIES 
Hr: | J. M. JAMESON, Editor 


ELEMENTARY ELECTRICAL TESTING 


BY 


VY. KARAPETOFF 


Cornell University 


A MANUAL FOR TECHNICAL HIGH SCHOOLS AND FOR EVENING CLASSES 
IN APPLIED ELECTRICITY AND ELECTRICAL MACHINERY’ 


JOHN WILEY & SONS, Ino., New Yorx 


Coprrigut, 1913, sy V. KaraPerorr 








Elect. Eng. Zi ab2o MeClurg 44 





NUMBER. 
E 200-1. 


E 201-1. 


E 201-2. 
E 201-3. 


E 202-1. 
E 203-1. 
E 203-2. 
E 203-3. 
E 204-1. 


E 205-1. 
E 205-2. 
E 206-1. 


THE LOOSE LEAF LABORATORY MANUAL 
THE WILEY TECHNICAL SERIES—J. M. Jameson, Editor 





ELEMENTARY ELECTRICAL 


TESTING 


By Pror. V. KARAPETOFF 
Cornell University 


CONTENTS 
TITLE. NUMBER. 
Calibration of a Commutator-type Watt- | E 206-2. 
hour Meter. 
E 207-1. 
Magnetization Curves of Iron and Steel. 

Hysteresis Loop. E 207-2. 
Influence of Air-gap in a Magnetic Circuit. | f 998-1. 
Influence of the Length and Cross-section of a E 208-2 

Magnetic Circuit on its Reluctance. 3 
Preliminary Study of a Direct-current Ma- BE 209-1. 
neg E 210-1. 
No-load Characteristics of a Shunt-wound 
Generator. 
Voltage Characteristics of a Shunt-wound | Hi 210-2. 
Generator. 
Excitation Characteristics of a Shunt-wound 
Generator. E 211-1. 
Load Characteristics of a Series-wound | E 212-1. 
Generator. 
Brake Test of a Shunt Motor. FE 213-1. 
Brake Test of a Series Motor. E 214-1. 
Ratio of Voltages and Currents in a Trans- 
former. E 215-1. 


43988 


TITLE. 
Load Tests on a Transformer. 


No-load Characteristics of an Alternator. 
Voltage Characteristics of a Loaded Alternator 
Starting an Induction Motor. 

Load Test on an Induction Motor. 
Charging a Storage Battery in Sections. 


Influence of Load and of Distance of Trans- 
mission on the Voltage Regulation of a 
Line. 


Influence of the Transmission Voltage and of 
the Cross-section of the Line on its 
Regulation. 


Starting Synchronous Motors. 


Assembling and Operating a Direct-current 
Switchboard. 


Test of a Lifting Electromagnet. 


Operating Motor-starters with No-voltage 
and Overload Release. 


Wiring a Machine-tool Controller. 






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THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


_ EXPERIMENT E 200-1. CALIBRATION OF A COMMUTATOR-TYPE WATT-HOUR METER 
WITH DIRECT CURRENT 


Apparatus.—Watt-hour meter; ammeter; voltmeter; stop-watch; load rheostat. 
Note.—An indicating wattmeter, if available, is preferable to an ammeter and a voltmeter. 


The purpose of the experiment is: (a) to adjust the brake magnets so that the meter reads 
correctly at the rated load; (b) to adjust the friction compensation in such a way that the meter . 
starts on as small a load as possible, without danger of creeping at no load; (c) to obtain a calibra- 
tion curve (with the best possible adjustments) showing the per cent of error at various loads. 

Connections.—The connections for the calibration are shown in the diagram; if an indicating 
wattmeter is used, its series coil is connected in place of the ammeter, and the shunt coil in place 





Connections for Calibrating a Watt-hour Meter. 


of the voltmeter. On small loads the power consumed in the voltmeter or in the shunt coil of 
the wattmeter may be appreciable, and may have to be added to the load. If the voltage is 
é, this loss is 727 =e?/r, where r is the resistance of the shunt circuit. 

Data Sheet.—The tabular record of data must contain columns marked seconds, revolutions, 
volts, and amperes (or watts). 

The Gear Ratio.—The reduction ratio of the gears between the meter shaft and the lowest 
recording dial is usually known. If not, it can be determined by a preliminary run, counting the 
number of revolutions of the disk necessary to move the lowest dial one division. This ratio 
is usually an even number such as 100, 200, etc. To save time, this test ought to be made on a 
heavy overload and possibly with one or more brake magnets removed, so as to run the meter 
as fast as possible. 

Full-load Adjustment.—Adjust the load rheostat so as to obtain a load nearly equal to the 
rated load of the meter. Keep the load constant, and count the number of revolutions of the 
armature shaft during a minute or so (use a stop-watch). Knowing the gear ratio, the error of the 
meter may be calculated. 

Suppose, for instance, that a constant load of 180 kw. was put on an integrating wattmeter 
and kept constant by means of an indicating wattmeter. Suppose that 15 revolutions of the 
armature disk be counted during 28 seconds, and that the reduction ratio of the recording gear be 
1000-1. If the value of one complete revolution of the pointer on the lowest dial is 100 kw.-hr., 


Copyright, 1913, by V. Karaprrorr., Published by Joun Witey & Sons, Inc. (OVER) 


the armature disk must complete one revolution while 100+1000 kw.-hrs., or 0.1 kw.-hr. is 
delivered to the load circuit. During the test, an energy equal to 180 X28 kw.-sec., or 
28 


cj So i he z 
80x 3500 1.4 kw.-hrs., 


has been delivered to the circuit. Therefore, the disk should have completed 
1.4+0.1=14 revolutions. 


In reality it made 15 revolutions; thus, at this particular load the meter runs about 7.1 per 
cent fast, and consequently registers 7.1 per cent more energy than is actually consumed. 

Adjust the brake magnets so as to obtain a more nearly correct speed, and repeat the run. 
After a few trials the adjustment can be made nearly perfect. 

Adjustment of the Friction Compensating Coil—In some meters the adjustment is made 
by moving the coil, in others by regulating the current through the coil. Adjust the meter so that 
the armature is ready to start on a very light load, but there is no danger of its creeping at no 
load. Sometimes a meter, correctly adjusted on a testing rack, begins to creep at no load, 
when put in service on a wall where it is subjected to jarring from the street or from an engine 
working near by. Therefore, jar the meter slightly while making the no-load adjustment, Also, 
try throwing off a heavy load suddenly and see if the meter stops in a short time. Determine at 
what lowest per cent of the rated load the meter can be made to start positively, without the 
danger of its creeping at no load. If a considerable adjustment of the compensating coil was 
necessary, check the full-load adjustment of the brake magnets. 

Calibration Curve.—Bring the load up to 25 to 50 per cent above the rating of the meter. 
Keep the load constant and count a certain number of revolutions of the meter shaft, with a 
stop-watch. The larger the number of revolutions counted, the more accurate will be the 
calibration. Reduce the load in steps, down to the smallest at which the meter runs positively 
and, at each step, repeat the calibration. Record all the readings in the data sheet. The calibra- 
tion curve ought to contain at least ten points. 

Before leaving the laboratory, note the make and the serial numbers of the instruments used 
and their correction constants if any. Inspect the meter and make a sketch of its principal 
parts. Make clear to yourself the precautions taken in the construction of the case so as to prevent 
tampering by dishonest persons. 

Report.—(1) Describe the meter tested and illustrate your description by neat sketches of 
details. 

(2) Give your calculations for full-load adjustment. 

(3) Describe your findings with regard to the light-load adjustment. 

(4) Plot the calibration curve, using per cent of rated load as abscissze and per cent “ slow ” 
or “ fast’ as ordinates. 

(5) Answer the following questions: 

(a) Is the calibration of the meter affected by reasonable fluctuations of voltage? 

(b) What is the higher limit of voltage on which a given meter may be operated with 
safety? 

(c) Will the meter run backwards if the line wires are interchanged? 

(d) If there is a suspicion that a dishonest customer tries to cheat the power supply 
company by tampering with the meter, how would you proceed in detecting and 
proving his guilt? 

(e) A meter is designed for 20 amp. current and has 8 turns in the series coils. How 
would you redesign the meter for 40 amp. capacity at the same voltage, withowe 
changing the armature or the gears? 


THE LOOSE LEAF LABORATORY 
ELECTRICAL TESTING 


EXPERIMENT E 201-1. MAGNETIZATION CURVES IN IRON AND STEEL. HYSTERESIS 
LOOP 


Apparatus.—Specimens of cast iron, electrical steel laminations, wrought iron, etc., in 
the form of rings provided with primary and secondary windings; ammeter; ballistic galvanometer; 
resistance box; adjustable rheostat; storage battery; double-pole, double-throw switch; fuse block. 

The purpose of the experiment is to obtain magnetization or B-H curves of the available 
specimens, the curves to be similar to the curve shown in Fig. 1. This curve gives the 
relation between the magnetic intensity, H, 
and the flux density, B, in the specimen. 
The magnetic intensity is to be expressed in ee! 
ampere-turns per cm. length of path, and 
the flux density in gausses. Steel and iron | 
retain some magnetization after the external 
magnetomotive force has been removed; for 
this reason the magnetization curve depends =H 50 
upon the preceding “ history ’”’ of the sample. 
When all traces of this history or “residual 
magnetism ”’ have been removed, the magnet- ook ; [5000 
ization curve begins at the origin, or B=0 for 
H=0. This curve, OM, Fig. 1 is called the M1 a eI 
neutral or the virgin curve of the material. : om = 8110000 ey = 

If, having reached a point M on the neutral Ae a : 
curve (OL =50 ampere-turns per em.), the cur- Fic. 1—Magnetization Curve and Hysteresis Loop. 
rent in the exciting winding be reduced, the 
values of B do not follow the curve MO, but follow the curve MR. When the exciting circuit 
is opened, the sample still possesses a flux of a density of about 4000 gausses. If now the 
exciting magnetomotive force be reversed, it takes between 7 and 8 ampere-turns per cm. to 
remove the residual magnetism. Increasing the magnetizing force in the negative direction to 
the value OL;=50 ampere-turns per cm., the point Mj, is reached, for which the flux density 
I4M,=LM=7000 gausses. If now the current be again reduced to zero and then increased 
in the opposite direction, the flux density follows the curve M,R1KiM, thus completing the 
“hysteresis loop.” For each point on the neutral curve there is a corresponding hysteresis 
loop similar to the above. 

The Ballistic Galvanometer.—A ballistic galvanometer is an instrument which, by its 


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Fig. 2.—Connections for the Test. 


deflection, measures a rapid electric discharge passed through its moving coil. Referring to 
the diagram of connections, Fig. 2, if the value of the flux in the specimen be quickly changed 


Copyright, 1913, by V. Karapetorr. Published by Joun Winey & Sons, Ine. 


by changing the resistance in the battery circuit, an electromotive force of short duration is 
induced in the secondary winding connected to the galvanometer. This electromotive force 
produces a “transient” current in the secondary circuit, so that there is a brief electric dis- 
charge through the galvanometer. Thus, a ballistic galvanometer may be made to measure 
variations in electric flux, utilizing the law of induction. 

Theory and experiment show that the deflection on the discharge is proportional to the 
change in flux. Moreover, with the same e.m.f. the instantaneous current is inversely proportional 
to the resistance of the secondary circuit (Ohm’s law), so that the total discharge is also inversely 
proportional to the total resistance of the galvanometer circuit (including the secondary winding, 
the resistance box and the resistance of the galvanometer coil). The induced e.m.f., and con- 
sequently the discharge, is proportional to the number of turns in the secondary winding. We 
thus have 


Discharge Q= C0 ee ll 


where © is the sudden change in flux, n is the number of turns in the secondary winding, R is the 
resistance of the secondary circuit, and Cy is a coefficient of proportionality. But, on the other 
hand, the discharge Q is also proportional to the deflection 6 as read on the scale, or Q=C26, 
where C2 is a constant. Eliminating Q from the two equations, and solving for ® we get 


BR 
a I 


where C is equal to Ci/C2 and is the galvanometer constant for flux. The constant C is either 
given, or may easily be determined by producing a known variation of flux, ®, through a coil 
connected to the galvanometer as explained under “ Calibration of the Galvanometer.”’ 

Method.—Connect the apparatus as in Fig. 2. The neutral curve and the hysteresis loop 
are obtained by varying the flux in steps. That is, beginning at the point O of the curve, the 
magnetomotive force is suddenly raised by a known amount, and the discharge through the 
galvanometer is observed. The flux &; is calculated by using formula (2). Let now the current 
be again increased by a certain amount and the discharge observed. Let the new flux varia- 
tion be zg. Then the total flux in the sample is 6;+ 42. Thus, by continuing this process in steps 
the magnetization may be carried to any desired point M. 

Starting now on the hysteresis loop, the current is reduced, so that the galvanometer deflects 
in the opposite direction, and the new values of ® are to be subtracted from the preceding sum. 
The purpose of the preliminary trials is to decide on the number and the approximate size of 
the steps in which the current is to be varied, to select the proper value of the resistance in the 
galvanometer circuit, and to acquire some skill in the handling and reading of the ballistic 
galvanometer. 

To Demagnetize the Sample.—Open the galvanometer circuit and bring the magnetizing 
current to the highest practicable value. Now keep on reversing the current by means of the 
double-throw switch and at the same time gradually reduce the current to zero by means of 
the rheostat. If this operation is carefully performed, the sample is practically neutral. The 
student must now be careful not to magnetize the sample until he is ready to begin the neutral 
curve. Moreover, once having started, it is not permissible to open the circuit or to reduce the 
current until the desired limit M is reached. Otherwise, the sample would be started on another 
hysteresis loop, and all the preceding readings lost. Should this happen by oversight, demagnetize 
the sample by reversals as above, and begin the readings anew from the point O. 

The Test Proper and the Data Sheet.—The test is conducted as explained in the preliminary 
trials, keeping in mind the precautions stated in the preceding paragraph. Record the amperes 
and the galvanometer throws, beginning with the point O and taking a complete hysteresis 
loop. Also note the values of the resistance plugged in the galvanometer circuit. 

Perform similar tests with the other kinds of steel and iron available, using the same maximum 
value of Hin all cases. Be careful to mark the readings which are negative. 


Before leaving the room, note the following data: The galvanometer constant, the number 
of turns in the primary and the secondary windings on the rings, the cross-section of the iron 
and the mean length of the path of the flux in the rings. 

Calibration of the Ballistic Galvanometer.—If the galvanometer constant C is not known, 
remove the specimen ring, and connect in its place the primary winding of a standardizing 
solenoid with an air core. This is a straight solenoid, the axial length of which is large as com- 
pared to the diameter of the air core. When such a solenoid is energized, the magnetic flux 
density inside it, near the middle portion, varies according to the theoretical relation 


oe 1.26 Ha, ° . . . * ° . . . ° . . . (3) 


where Bz is in gausses, and Ha=7nyi, is the magnetic intensity in ampere-turns per centimeter 
length. In this expression 7 is the current in amperes and n; is the number of primary turns per 
centimeter length. Knowing Ba and the cross-section A of the air core in square centimeters, 
the flux inside the solenoid is determined from the formula 


Se= Bil = ae en eects g(t) 


Thus, in order to find the constant C of the galvanometer, place a secondary winding of a 
known number of turns over the middle portion of the standardizing solenoid and connect this 
secondary winding to the ballistic galvanometer. Bring up the current to a desired value (with 
the galvanometer circuit open), close the galvanometer circuit and open the primary circuit. 
Note the deflection of the galvanometer, the value of the primary current, and the resistance 
of the secondary circuit. Also determine the values of ni and A for the solenoid. With these 
data, using equations (3) and (4), the flux 2 becomes known, and consequently C can be cal- 
culated from equation (2). It is advisable to take several readings, using different values of cur- 
rent so as to eliminate possible errors of observation, and to increase the accuracy of the 
result. 

Report.—(1) Plot B-H curves on one and the same sheet of cross-section paper, and to 
the same scale, so as to bring out clearly the difference in the properties of the samples investigated. 

(2) On the same sheet plot equation (3) for the air, so as to show how much more “ per- 
meable ” iron is than the air. 

(3) For at least one sample, plot a curve of relative permeability against values of B in iron 
as abscisse. Relative permeability is defined as 


whats 
By’ 


in other words, it is the ratio of flux density in iron to that in the air, for the same value of H. 
Use values of B from the neutral curve. 
(4) If you had to determine your own galvanometer constant, give the data and show coe 
you found the result. 
(5) Answer the following questions: 
(a) What would be the shape of the magnetization curve and of the hysteresis loop 
if the sample were not thoroughly demagnetized at the beginning of the test? 
(b) What would be the shape of these curves if during the test the current were 
reduced and then increased again, due to a wrong manipulation? 
(c) Why does the ballistic galvanometer give wrong indications when the flux varies 
slowly instead of suddenly? 
(d) Explain why if the rings were not uniformly wound but the primary winding were 
concentrated at one place of the ring, an error would be introduced due to 
magnetic leakage through air. 





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THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 201-2. INFLUENCE OF AIR-GAP IN A MAGNETIC CIRCUIT 


Apparatus.—A magnetic circuit with adjustable air-gaps, such for example, as that 
shown in Figs. 1 and 2; exciting coils for the same; a secondary coil for the same; ammeter; 
ballistic galvanometer with a resistance box (multiplier); rheostat for the exciting circuit; 
double-pole double-throw switch; fuses. 

The purpose of the experiment is to investigate the harmful effect of an air-gap ina 
magnetic circuit which consists largely of iron. The presence of an air-gap reduces considerably 
the flux with a given magnetomotive force; much more excitation is required, therefore, in 
order to obtain the same flux as without the air-gap. For this reason, in electrical machinery 
and apparatus, air-gaps are avoided or their length is reduced to a possible minimum. This 
experiment must be preceded by Experiment E 201-1 on “‘ Magnetization Curves in Iron,” 
and the student is assumed to be familiar with the method and the explanations given there. 


Ball Ga. Multiplier(Resistance) 







D.C. Supply 


hs Variable 
G 


Double 
-throw 
Switch 
































Fic. 1.—Magnetic Circuit with Fig. 2.—Diazram of Connections. 
Adjustable Air-gap. 


Connections.—The diagram of the required connections is shown in Fig. 2. The exciting 
coils are concentrated symmetrically near the air-gap, so as to avoid magnetic leakage which 
would vitiate the results. 

Data Sheet.—Record the exciting amperes, galvanometer deflections, the resistance in 
the galvanometer circuit, and the length of the air-gaps. 

Readings.—Take magnetization curves of the apparatus with various known values of the 
air-gap. Before beginning the first run, make a few preliminary trials so as to select proper values 
of the steps in changing the current and of the resistance in the galvanometer circuit. If 
desired, the method of reversals may be used instead of that of increasing the current step by 
step. Beginning with zero current, increase the current to say 2 amp. by closing the switch 
with the proper resistance in the exciting circuit, and read the galvanometer deflection. Now 
open the galvanometer circuit and slowly increase the current to say 3 amp. Close the galva- 
nometer circuit and quickly reverse the double-throw switch. The flux is changed from +4 to—9, 
so that the change in flux is 26. This value must be used in formula (2), Experiment E 201-1. 


Copyright, 1913, by V. KaraprTorr. Published by Joun Witey & Sons, Ine. (OVER) 


Now raise the current to say 4 amp., and again reverse rapidly. With this method, twice the 
total flux is measured in each case, and not the additional flux as in the step-by-step method. 
When the galvanometer deflections increase beyond the limit of the scale, more resistance is to be 
connected in the galvanometer circuit, so as to reduce the sensitiveness of the instrument. 

(a) After the preliminary trials eliminate the air-gaps as much as possible by bringing the 
iron cores together, and demagnetize the iron cores thoroughly. Take a magnetization curve 
and again demagnetize the circuit. 

(b) Repeat the test with a very small but definite air-gap, say 0.2 mm. Such an air-gap 
is obtained by interposing a piece of paper of fiber between the iron cores; these materials being 
non-magnetic have the same effect as so much air. 

(c) Make similar tests with larger air-gaps. 

Before leaving the room, measure the dimensions of the cores, ask about the number of 
turns in the coils, find out the galvanometer constant, and the correction of the ammeter if any. 

Report.—(1) Plot curves of flux against exciting ampere-turns, one curve for each value 
of the air-gaps. 

(2) Plot curves of flux densities against the ampere-turns required for the air-gaps alone. This 
is done by subtracting from the total ampere-turns those required for the iron parts. For instance, 
let 200 ampere-turns be required for a total flux of say 30,000 lines when the air-gap is 0.5 mm., and 
40 amp.-turns for the same flux with no air-gap. Then 200—40=160 amp.-turns are required for 
the air-gaps. Theoretically these curves are straight lines and must show that the air-gap 
ampere-turns are proportional to the length of the gap and to the flux density. Check this rela- 
tion and explain the sources of discrepancy and inaccuracies if any. 

(3) Check equation (3), given in Experiment E 201-1, “Magnetization Curves,” as follows: 
Let the air-gaps in one of the tests be 0.4 mm. each, or together 0.8 mm. At a flux density of say 
3000, the intensity in the air-gaps is H.=3000/1.26=2390 amp.-turns per cm. Hence, the 
ampere-turns required for the two air-gaps are 2390X0.08=191. From your curves you should 
find approximately this value. Check this relation, for several of your curves, and explain 
any possible sources of discrepancy. 


, 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 201-3. INFLUENCE OF THE LENGTH AND CROSS-SECTION OF THE 
MAGNETIC CIRCUIT ON ITS RELUCTANCE 


Apparatus.—Four or six U-shaped rectangular iron cores similar to those shown in the 
sketch below; the same number of straight pieces of the same cross-section that may be put between 
the abutting ends of the U-pieces; exciting coils; secondary coils; ammeter; ballistic galvanometer 
with a resistance box (multiplier); rheostat for the exciting circuit; double-pole, double-throw 
switch; fuses. 

The purpose of the experiment is to prove that the number of ampere-turns required to 
produce a certain flux is proportional to the length of a uniform magnetic circuit; also to show that 
the same number of exciting ampere-turns can be made to produce a much larger flux by 
increasing the cross-section of the magnetic circuit. This experiment must be preceded by 
E 201-1, ‘‘ Magnetization Curves in Iron,” and the student is assumed to be familiar with the 
method and the explanations given in that exercise. 

Connections.—A diagram of connections for the experiment is shown in Fig. 2. The 


Ball Ga. 


Multiplier(Resistance) 






Double 
-throw 
Switch 



































Fig. 1.—Adjustable Magnetic Circuit. Fia. 2.—Diagram of Connections. 


exciting coils must be distributed as nearly as possible over the length of the circuit so as to 
avoid magnetic leakage which would vitiate the results. 

Data Sheet.—Record exciting amperes, galvanometer deflections and the resistance in the 
galvanometer circuit. Mark the number of pieces in series and in parallel. 

Readings.—Take magnetization curves of the apparatus, varying the length and the 
cross-section of the circuit by adding pieces in series and in parallel. Before beginning the 
first run make a few preliminary trials so as to select proper values of the steps in changing 
the current and of the resistance in the galvanometer circuit. If desired, the method of reversals 
may be used instead of increasing the current step by step. Beginning with the zero current, 
increase the current to say 2 amp. by closing the switch, and read the galvanometer deflection. 
Open the galvanometer circuit and slowly increase the current say to 3 amp. Close the 
galvanometer circuit and quickly reverse the double throw switch. The flux is changed from 


Copyright, 1913, by V. Karaperorr, Published by Joun Witey & Sons, Inc, (OVER) 


--+6 to—®, so that the change in flux is 26. This value must be used in formula (2) of Exp. 
FE 201-1. Now raise the current to say 4 amp., and again reverse rapidly. With this method 
twice the total flux is measured in each case, and not the additional flux as in the step-by-step 
method. When the galvanometer deflections increase beyond the limit of the scale, more resistance 
is to be connected into its circuit, so as to reduce its sensitiveness. 

(a) After the preliminary trials, place two U-shaped pieces in the exciting coils, press them 
tight together end to end, and demagnetize the circuit thoroughly. Take a magnetization 
curve and again demagnetize the circuit. 

(b) Repeat the test with two sets of U-shaped pieces connected magnetically in parallel 
and excited by the same coils; also with three pairs of pieces if such are available. 

(c) Take again two U-shaped pieces and interpose two straight rods between them so as to 
form a longer circuit. Clamp the whole tightly together and demagnetize the circuit thoroughly. 
Take a magnetization curve and then demagnetize the circuit. 

(d) Repeat the preceding test, using four U-shaped pieces and four straight pieces. 

Before leaving the room, measure the dimensions of the cores, ask about the number of 
turns in the coils, find out the galvanometer constant, and the correction of the ammeter if any. 

Report.—(1) Plot curves of flux against exciting ampere-turns, one curve for each combina- 
tion of the cores. 

(2) Show from these curves that, at a certain flux density, the number of ampere-turns 
required is proportional to the length of the circuit and does not depend upon its cross- 
section. 

(3) Show that the number of ampere-turns necessary to produce a given flux increases 
faster than the decrease in the cross-section of the magnetic circuit; explain how this fact is due 
to saturation in the iron. 

(4) The ratio of the magnetomotive force to the flux is called the reluctance of the magnetic 
circuit. It is analogous to the resistance of an electric circuit, the resistance being the ratio of 
the electromotive force to the current. Show from the results of your experiment that, neglecting 
saturation, the reluctance is directly proportional to the length of the magnetic circuit and is 
inversely proportional to its cross-section. A similar relation holds for the resistance of an 
electrical conductor. 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 202-1. PRELIMINARY STUDY OF A DIRECT-CURRENT MACHINE 


Apparatus.—Shunt-wound machine to be studied and a suitable drive; field ammeter; 
main ammeter; voltmeter; field rheostat; load rheostat; starting rheostat; double-pole switch 
and fuses for the main circuit (or a circuit breaker); single-pole switch for the field circuit; speed 
counter (or tachometer). 

The purpose of the experiment is to familiarize the student with the structural details and 
the principal operating features of a direct-current machine. The experiment consists of three 
parts: making drawings and specifications of the machine; running it as a generator; and 
running it as a motor. The experiment may profitably be extended over two laboratory periods, 
or the part referring to the operation as a motor may be omitted until the student is ready to take 
up the brake tests on direct-current motors (Experiments E 205-1 and E 205-2). 

Drawings and Specifications of the Machine——Assume the machine to be bisected bya 
vertical plane through the geometrical axis of the shaft. Make a sketch of what would be visible 
in such a longitudinal section of the machine, naming the parts and marking the dimensions, 

Then assume the machine to be bisected by a vertical plane perpendicular to the shaft, 
and passing through the armature. Make a sketch as before of the principal parts as they would 
appear in such a transverse section. 

In addition to the sketches called for in the above sections, make sketches of the commutator, 
the brushes and brush holders, the armature coil, and such other parts as do not show well in the 
principal drawings. 

Ascertain the following data, in addition to those copied from the name plate: 

Field.—Number of poles; material of the frame; material of the pole pieces, whether solid 
or laminated, bolted on or cast with the frame. Measure the resistance of the field winding. 

Armaiure.—‘ Lap’ wound or ‘“ wave” wound; wire or strap used for the armature con- 
ductors; armature core smooth or ‘slotted, solid or laminated; armature coils held in place by 
means of wedges or by binding wires; number of ventilating ducts provided; armature stampings 
assembled on the shaft or on a “spider” keyed to the shaft. Measure the armature resistance 
from brush to brush. 

Commutator.—Number of segments and their material; how they are insulated from one 
another; how the segments are held in place. 

Brushes and brush holders——Number of studs; number of brushes per stud; material of the 
brushes; provision for adjusting the brush: pressure; provision for adjusting the position of the 
brushes on the commutator. 

Bearings.—Provisions for lubrication. 











Voltmeter 


Main Ammeter 


Fiela 
Windings 





Fuses or 
Circuit Breaker 


_ Field Rheostat 
Fria. 1.—Connections as a Generator. 


Operation as a Generator.—(1) See that the brushes are in good condition and properly 
placed, and that the machine is well oiled. Connect up for running as in Fig. 1, putting a suitable 


Copyright, 1913, by V. Karaprerorr, Published by Jonn Witny & Sons, Ino, (OVER) 


ammeter and regulating resistance in the field circuit, a second ammeter in the external circuit, 
and a voltmeter across the terminals. Place switches in both the external and the field circuits. 
Have your connections approved by an instructor before starting the machine. 

(2) Open the switches in both the external and the field circuits, then drive the generator 
at its rated speed. Observe the voltmeter reading. This is the e.m.f. induced by the rotation of 
the armature coils in the weak magnetic field due to the residual magnetism of the field cores. 
Try the effect of change of speed upon the e.m.f. Send a small current from an external source 
through the field windings and note the effect upon the induced e.m.f. 

(3) Insert the maximum resistance of the field rheostat, and leave the switch to the external 
circuit open. Close the field switch. The induced e.m.f. due to the residual magnetism will 
cause a small current through the field coils, which, if in the proper direction, will magnetize the 
field coils in the same direction as the residual magnetism. This stronger field in turn induces a 
stronger e.m.f., and therefore a greater field current, etc., and the machine will “ build up.” Note 
the increase in voltage at the brushes, as indicated by the voltmeter, and the rise in field current, 
as indicated by the field ammeter, during this process. If the field coils are not properly con- 
nected to the armature terminals, the flux due to the field current will weaken the residual magnetism 
and the machine cannot build up. In this case, the connections to the brushes must be reversed. 
Decrease the field resistance until the voltmeter indicates the normal voltage, at normal speed. 
Why does decreasing the field resistance increase the pressure at the brushes? 

(4) Close the main switch and reduce the resistance of the load gradually until the full load 
is reached. What effect has increasing the current supplied by the machine upon the voltage 
at the brushes? Why? Bring the voltage to the normal value by adjusting the field rheostat, 
and run the machine at its normal voltage and a apeest Determine the load current and compute 
the output of the generator in watts. 

Operation as a Motor.—(1) Connect up as in Fig. 2; do not use any switch in the field 

circuit, because if this switch should be opened by 
ete an oversight, the motor may acquire a dangerous 
nae Hises speed (run away). 

(2) Have all of the starting resistance “ in” 
and all of the field resistance ‘‘ out.’”’ Close the 
main switch and, as the motor starts, cut the start- 
ing resistance out in steps until the armature is con- 
nected directly across the line. 

(3) To stop the motor, open the main circuit 
and then connect all of the starting resistance in 

Fic. 2.—Connections for Operation as a Motor. the circuit so as to prepare the motor for the next 
run. Always see to it that the armature is protected 
by the starting resistance when the motor stands still. 

(4) Reverse the armature terminals and start the motor in order to see that its direction of 
rotation is reversed. ‘Try the same with the field terminals. Finally, reverse the line terminals 
to find that the motor continues to run in the same direction. Explain the reasons for each of the 
preceding. 

(5) Having brought the motor up to its speed, slowly reduce the field current by means of 
the field rheostat, and note that the speed increases. Take a few readings so as to plot a curve 
of field current vs. speed. Ask the instructor about the safe maximum speed of the machine. 
If the brush rigging permits, try to regulate the speed by shifting the brushes. 

Report.—(1) Make neat drawings of the machine, approximately to scale. 

(2) Describe the construction of the machine. 

(3) Give data and answer the questions asked under ‘‘ Operation as a Generator.” 

(4) Report your findings from the tests called for under “ Operation as a Motor.” 





o 
Starting Rheostat 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 203-1. NO-LOAD CHARACTERISTICS OF A SHUNT-WOUND GENERATOR 


Apparatus.—Shunt-wound machine with a suitable drive; field ammeter; voltmeter; field 
rheostat; switch for the field circuit; speed-counter (or tachometer). 

The purpose of the experiment is to investigate the relation between the exciting current 
and the terminal voltage of a shunt-wound machine at no-load. Since the induced voltage at 
a constant speed is proportional to the useful flux of the machine, the experiment is also a study 
of the magnetic circuit of the machine. It is advisable to have the experiment preceded by 
E 202-1 “ Preliminary Study of a Direct-current Machine.” 

Connections.—Connect the apparatus as in the figure. If the polarity of the brushes is 
known, connect the instruments so as to read in the right direc- 
tion; if not, start the machine and bring it up to a speed sufficient 
to read the voltage between the brushes, so as to determine their 
polarity with a voltmeter. Have your connections approved by 
an instructor before starting the machine. 

Data Sheet.—Record terminal volts, field amperes and the  rieia 
speed of the machine. bape 

Readings.—(1) Bring the machine up to its rated speed and 
excite the field. Begin readings with the highest possible value of 
the field current, in other words, with the field rheostat short- 
circuited. Read field amperes, volts and the speed of the machine. 
Reduce the field current in steps, and at each step take similar 
readings. The value of the residual magnetism at the begin- 
ning of the experiment is rather indefinite; therefore, it is advisable to begin the excitation at its 
maximum and reduce to zero. After this, take readings with an increasing field current, in 
order to see the influence of residual magnetism. This is somewhat analogous to taking a hys- 
teresis loop (Experiment E 201-1). 

(2) The induced voltage is proportional to the speed of the machine when the field current 
is kept constant; this is according to the fundamental law of induction. To prove this exper- 
imentally, select a field excitation and run the machine within as wide a range of speed as the 
driving motor will permit. Keep the exciting current constant by regulating the field rheostat, 
or excite the machine from a separate source. Repeat this run with two or three different values 
of field current. 

(3) Investigate the ability of the machine to excite itself. Run it at the rated speed, and 
find the rheostat notch on which the field just begins to build up. Measure the corresponding 
field current, the final voltage, and the number of seconds of time from the closing of the field 
switch to the moment when the voltage reaches its final value. Repeat the same experiment 
with less resistance in the field rheostat, and at different speeds of rotation. 

Report.—(1) Plot the no-load saturation curve, that is, terminal volts against field 
current as abscisse, at the rated speed. Where the speed was not exactly right, correct the 
voltages in direct proportion. 

(2) Plot curves which show that the induced voltage is directly proportional to the speed. 

(3) Give the numerical results on self-excitation. 

(4) Answer the following questions: 

(a) Would the machine excite itself if driven in the opposite direction? 

(b) If the residual magnetism is too weak to build up the field at the normal speed, would 
it help the matter to speed up the machine and then return to the rated speed? 

(c) Why is the lower portion of the no-load characteristic practically a straight line? 

(d) Would the no-load characteristic be affected if the brushes be shifted from the neutral 
position? 
Copyright, 1913, by V. Karaprrorr, Published by Joun Witny & Sons, Ine. 


Field Ammeter Voltmeter 





Field Rheostat 
Connections for the Test. 





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THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 203-2. VOLTAGE CHARACTERISTICS OF A SHUNT-WOUND GENERATOR. 


Apparatus.—Shunt-wound generator with a suitable drive; main ammeter; field ammeter; 
voltmeter; load rheostat; field rheostat; double-pole switch and fuses for the main circuit (or 
a circuit-breaker); single-pole switch for the field circuit; speed-counter (or a tachometer). 

The purpose of the experiment is to investigate fluctuations in the terminal voltage of the 
machine caused by fluctuations in the load. It is assumed that the machine is left without atten- 
tion, so that no adjustments of the field current are made in order to keep the voltage constant 
when the load varies. In some cases a generator operates under nearly such conditions in 
practice. The conditions are opposite to those in experiment E 203-3, “Excitation Characteris- 
tics of a Shunt-wound Generator,’ where it is assumed that the field current of the machine is 
being regulated all the time so as to keep the voltage constant. This experiment must be pre- 
ceded by experiments E 202-1, “ Preliminary Study of a Direct-current Machine,” and E 203-1, 
“No-load Characteristics of a Shunt-wound Generator,” or at least by one of them. 












Voltmeter 





Main Main Ammeter 


Switch 


Field 
Windings 





Fuses or 
Circuit_ Breaker 


Field Rheostat 


Connections.—Connect the apparatus as in the diagram. If the polarity of the brushes is known, 
connect the instruments so as to read in’the right direction; if not, start the machine at no-load 
and bring it up to a speed sufficient to read the voltage between the brushes, so as to determine 
their polarity with a voltmeter. Leave both switches open, and set both rheostats at a maximum 
of resistance. Have your connections approved by an instructor before starting the machine. 


Data Sheet.—Record your readings on a data sheet similar to the one shown below. Before 
beginning the readings, copy the name-plate of the machine and note the make and the serial 
numbers of the measuring instruments, also their correction constants if any. The speed must 
be kept constant during the whole test, but small, unavoidable deviations must be recorded on 
the data sheet. 


Data Sheet 


mMormalrvorace ttle Ms PS tee ee reset KR load. 












Field Amps. Load Amps. 


Instrument No......... 


SSS ee eee ee 


DIOUBLATING Goose cs fee. 


Ee ee eee 


——S—[$ | f | es 





Copyright, 1913, by V. Karaprtorr. Published by Joun Wixey & Sons, Inc. (OVER) 


Readings.—Bring the machine up to its rated speed, close the field switch, and excite the 
field to some value below normal. Close the main switch and carefully adjust the rheostats in such 
a way as to finally obtain the rated load current at the rated terminal voltage and at the rated 
speed. Record these readings, also the field current, and leave the field rheostat in this position 
for the rest of the run. By regulating the load rheostat, increase the current to about 25 per cent 
above the rated current and adjust the speed of the machine to its correct value. Take readings 
of volts, load amperes, and field current. Now decide as to the number of points desired on the 
curves and the approximate values of the load current for which readings are to be taken. For 
instance, if the rated current of the machine is 100 amps., the readings would begin at 125 amps., 
and a good curve would be obtained by reducing the load to 110, 100, 80, 60, 40, 20 amps., and 
finally to zero by opening the main switch. If time permits, readings should be taken at closer 
intervals, because there is always a possibility of one or two readings being off on account of some 
unnoticed error or inaccuracy. 

If circumstances permit, take another set of readings corresponding to some different initial 
conditions; for example, a different voltage at full load, a slightly different speed, a different posi- 
tion of the brushes, etc. Such tests help to form a more complete picture of the performance and 
the properties of the machine. If requirement (3) of the report is specified, measure the resist- 
ance of the armature, including that of the brushes. 

Report.—(1) Plot to armature amperes as abscissae the following quantities: terminal volts, 
field amperes, load amperes, and the output in kilowatts. The armature current is the sum 
of the current in the load circuit and that in the field windings. The output is equal to terminal 
volts times terminal amperes divided by 1000. 

(2) Figure out the per cent voltage regulation of the machine or 


Ei; =Ep 
10x, 


where £; is the terminal voltage at the rated load and Epo the no-load voltage. It is understood 
in the definition that the speed and the setting of the field rheostat (not the field current) are 
the same at no-load as at full-load. 

(3) Supplement the curves by a curve of the induced e.m.f. The induced e.m.f. is equal to 
the terminal voltage increased by the amount, tra, of voltage drop in the armature, where i, is 
the armature current, and 7, the resistance of the armature including that of the brushes. 

(4) Plot on the same curve sheet a curve of the voltage induced at no-load, at the corre- 
sponding values of field current. The difference between the curves (4) and (3) is a measure of 
the armature reaction. 

(5) Answer the following questions: 

(a) What are the three causes of voltage drop in a loaded generator, as compared to the 
no-load voltage? 

(6) Why should the resistance of the shunt winding be high as compared to that of the 
armature? 

(c) What is meant by “ armature reaction ” ? 

(d) What means are employed in practice to keep the terminal voltage approximately constant 
with varying load? 

(e) Why is the load switch kept open until the field has been built up? 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 203-3. EXCITATION CHARACTERISTICS OF A SHUNT-WOUND 
GENERATOR 


Apparatus.—Shunt-wound generator with a suitable drive; main ammeter; field ammeter; 
voltmeter; load rheostat; field rheostat; double-pole switch and fuses for the main circuit (or a 
circuit-breaker); single-pole switch for the field circuit; spced-counter (or tachometer). 

The purpose of the experiment is to investigate the necessary variations in the field current 
in order to keep the terminal voltage of the machine constant when the load varies. In some 
cases a generator operates under nearly such conditions of constant attention in practice. The 
conditions are opposite to those in experiment E 203-2 “ Voltage Characteristics of a Shunt- 
wound Generator,” where it is assumed that the machine is left without attention. This exper- 
iment must be preceded by Experiments E 202-1 “‘ Preliminary Study of a Direct-current Machine ” 
and E 203-1 ‘“ No-load Characteristics of a Shunt-wound Generator,” or at least by one of them. 

Connections.—Connect the apparatus as in the diagram. If the polarity of the brushes is 
known, connect the instruments so as to read in the right direction; if not, start the machine at no 








Voltmeter 






Main Ammeter 
Load 


Main 
Switch 






Field 
Windings 


Fuses or 
Circuit Breaker 


Field Rheostat 


load and bring it up to a speed sufficient to read the voltage between the brushes, so as to 
determine their polarity with a voltmeter. Leave both switches open, and set both rheostats 
at a maximum of resistance. Have your connections approved by an instructor before starting 
the machine. 

Data Sheet.—Record your readings on a data sheet similar to the one shown below. 
Before beginning the readings, copy the name plate of the machine and note the make and the 
serial numbers of the measuring instruments, also their correction constants if any. The speed 
and the voltage must be kept constant during each run, but small, unavoidable deviations must 
be recorded on the data sheet. 





- Load Amps, Field Amps. Volts. Speed. 






Inst. No. 











Readings.—Bring the machine up to its rated speed, close the field switch, and excite the 
field to some value below normal. Close the main switch and adjust the rheostats in such a way 


| Copyright, 1913, by V. Karapetorr, Published by Jon Witzy & Sons, Inc. (ovER) 


as to obtain finally the rated terminal voltage and a load current of about 25 per cent in excess 
of the rated current of the machine. Record these readings. Now decide as to the number of 
points desired on the curves and the approximate values of the load current for which readings are 
to be taken. For instance, if the rated current of the machine is 100 amps., the readings would 
begin at about 125 amps., and a good curve would be obtained by reducing the load to 110, 100, 
80, 60, 40, 20 amp., and finally to zero by opening the main switch. If time permits, readings 
should be taken at closer intervals, because there is always a possibility of one or two readings 
being practically valueless on account of some unnoticed error or inaccuracy. 

If circumstances permit, take another set of readings corresponding to a different terminal 
voltage. At a higher voltage, the machine is more highly saturated and the per cent variation 
in field current is less, although the field current itself is greater. At a low voltage the field cur- 
rent is small but its per cent variation is much greater. Such tests help to make a more complete 
picture of the performance and the properties of the machine. If requirement (3) of the report 
is specified, measure the resistance of the armature including that of the brushes. 

Report.—(1) Plot to armature amperes as abscisse, the following quantities: terminal 
volts, field amperes, load amperes and the output in kilowatts. The armature current is the 
sum of the current in the load circuit and that in the field winding. The output is equal to 
terminal volts times terminal amperes divided by 1000. 

(2) Figure out the per cent field current regulation of the machine, defined as, 


100x2—*, 
U1 
where 7; is the field current at full load and 7, that at no load. It is understood in this definition 
that the speed and the voltage are the same at no load as at full load. 

(3) Supplement the curves by a curve of the induced e.m.f. The induced e.m.f. is equal 
to the terminal voltage increased by the amount of voltage drop, zara, in the armature, where 7 is 
the armature current, and 7a the resistance of the armature including that of the brushes. 

(4) Plot on the same curve sheet a curve of the voltage induced at no load, at the correspond- 
ing values of the field current. The difference between curves (4) and (3) is a measure of the 
armature reaction. ~ 

(5) Answer the following questions: 

(a) What are the two causes for which it is necessary to increase the field current with 
the load, in order to keep the terminal voltage constant? 

(b) Why should the resistance of the shunt-winding be high as compared to that of the 
armature? 

(c) What is meant by “ armature reaction?” 

(d) What means are employed in practice to keep the terminal voltage approximately 
constant with varying load? 

(e) Why is the load switch kept open until the field has been built up? 

(f) Why is per cent field current regulation smaller in a highly saturated machine? 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 204-1. LOAD CHARACTERISTICS OF A SERIES-WOUND GENERATOR 


Apparatus.—Series-wound generator with a suitable drive; ammeter; voltmeter; load 
rheostat; double-pole switch and fuses for the main circuit (or circuit breaker); speed-counter 
(or tachometer). 

The purpose of the experiment is to investigate the relation between the load current and 
the terminal voltage of a series-wound generator. While such a generator is seldom used in 
practice, its characteristics are of interest because many shunt-wound generators are provided with 
an additional series winding and are then called compound-wound generators. 

Connections.—Connect the load-rheostat across the terminals of the machine in series with 
the main switch and the ammeter. Connect the voltmeter across the terminals of the machine. 
Have your connections approved by an instructor before starting the machine. 

Data Sheet.—Record volts, amperes and speed. The speed must be kept constant, but 
small unavoidable deviations must be recorded on the data sheet. Ifa run is to be made with the 
field weakened (see below) provide a separate column for the field current. 

Readings.—Bring the machine up to its rated speed with the main switch open. Cut in 
as much of the load resistance as possible and close the main switch. Carefully cut the resistance 
out and build up the voltage of the machine. Go up to the safe limit of the current and take 
the first set of readings at this limit. Decide upon the number of steps, gradually cut the 
resistance out, and take the readings at each step. At least ten steps are necessary in order to 
obtain a satisfactory curve. Having reduced the current to zero, increase the load again and take 
another load curve with the current increasing, so as to find out the effect of the residual magnetism. 

The load characteristics of a series-wound generator may be varied by shunting part of the 
armature current around the field. Connect a high-resistance rheostat across the field winding 
of the machine and insert a second ammeter in series with the field. Adjust the field rheostat 
so as to have the field current equal to say 80 or 90 per cent of the armature current, and take a 
load curve similar to the one taken before. 

Report :— (1) Plot to armature amperes as absciss, the terminal volts and the output in 
kilowatts. 

(2) If a run has been made with the field weakened, plot the results on the same curve sheet, 
viz., the field current, terminal volts and the output in kilowatts. 

(3) Answer the following questions: 

(a) Why is a series-wound generator not suitable for ordinary light and power circuits? 

(b) Why should the resistance of the series winding be low while that of shunt winding 
is high? 

(c) What would you do if the machine failed to excite itself, due to lack of residual mag- 
netism or to a residual magnetism of the wrong polarity? 

(d) Explain the shape of the voltage curve with a weakened field as compared to that 
with a full field. 


Copyright, 1913, by V. KaraPEToFF. Published by Jonn Witney & Sons, Inc. 








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THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 205-1. BRAKE TEST ON A SHUNT MOTOR 


Apparatus.—Shunt-wound motor; starting rheostat; field rheostat; Prony brake; main 
ammeter; field ammeter; voltmeter; switch and fuses for the main circuit (or circuit breaker); 
speed counter (or tachometer). 

The purpose of the experiment is to investigate the electrical characteristics of a shunt-wound 
motor similar to those shown in Fig. 1. 
Such characteristics are important for the 
prospective user of a motor, in order to 
determine if a motor offered for sale is 
suitable for his purpose. This experi- 
ment should be preceded by Experiments 
E 202-1 “ Preliminary Study of a Direct 






1 
nv efficiency : 





Main Switch 
and Fuses 













FYfeld current 
| 

=A 
| 


Shunt 
Field ist 

; | 
- | 
| 
Horse power output = ! 

? ! Starting Rheostat 

Fic. 1.—Electrical Characteristics of a Shunt-wound Motor.’ Fig. 2.—Connections for the Test. 


Current Machine” and E 203-1 “ No load Characteristics of a Shunt-wound Generator ”’ or at 
least by one of them. 

Connections.—Connect the apparatus as shown in Fig. 2; have all of the field resistance cut 
out and the whole starting resistance connected in. Have your connections approved by an 








Handwheel 





Cooling Water 








Fic. 3—Prony Brake. 


instructor before starting the machine. Start the machine slowly in order to see if it runs in 
the proper direction for the Prony brake. If not, reverse either the field or the armature 
terminals; look out for the beam of the brake that it does not strike you. 

Data Sheet.—Record your readings on a data sheet similar to the one shown below. 
Before beginning the readings, copy the name plate of the machine and note the make and the 
serial numbers of the measuring instruments, also their correction constants, if any. The field 
current must be kept constant during each run, but small unavoidable deviations must be recorded 
on the data sheet. 


Copyright, 1913, by V. Karaperorr. Published by JoHN Witey & Sons, Inc. (OVER) 





Pi hs ae as ee ae 
Arm. Amps. Field Amps. | Volts. Revs. per Min. pete ils 
| : 


Inst. No. 











——— ee ee ne eS |S ee 











Prony Brake.—The simplest device for loading motors is a Prony brake, a form of which 
is shown in Fig. 3, in side view and in cross-section. It consists of an iron band, ab, lined with 
soft wood or with heavy canvas. The band embraces the pulley of the motor, and is fastened 
to a beam, the other end of which rests on a scale. 

When the motor revolves, friction is developed between the lining of the brake and the pulley; 
the power of the motor is thus converted into heat. The brake pressure is regulated by a hand- 
wheel, and in this way any desired load is obtained. The turning moment, or the torque, as it is 
called, is measured on the scale. 

In most cases it is necessary to carry away the heat developed by friction, in order to pre- 
vent burning of the brake lining. The pulley, shown in the sketch, is cooled by a stream of water 
from the pipe w (see cross-section to the left); the water is thrown by centrifugal force against the 
inner surface of the face of the pulley and the flange prevents it from being spilled. 

If P is the net pressure in pounds on the scale at the end of a lever / feet long, the pressure 
at the end of a lever one foot long is Pl lbs.; consequently the work in foot-pounds, performed 
during one minute, is Pl X27n, where nis the number of revolutions of the motor per minute. The 
same in horse-power is 

PILX2rn 
33000 ’ 


or horse-power =——~ 
5252" 

The net pressure is obtained by subtracting from the actual scale reading the pressure exerted 
upon the scale when the motor is stationary with the power off. This initial pressure simply 
represents the unbalanced weight of the brake. Move the arm up and down to eliminate the 
friction in the bearings of the motor when obtaining this initial pressure. 

In the metric system, if P is in kilograms and / in meters, the formula is 


PLX2nn/60X9.81 _Pln 
1000 973 


Readings.—Before starting the motor, determine the initial pressure of the brake arm 
upon the scale, as explained above. Begin the test with the highest load, say 25 per cent overload; 
read armature amperes, field amperes, terminal voltage, speed, and brake load. Then gradually 
reduce the load to zero, taking a sufficient number of readings (from 8 to 10) for plotting curves. 
The field current and the terminal voltage must be kept constant throughout the whole test. 

Take a few points with the field current 10 and 20 per cent below normal. Also make runs 
at a line voltage, say 10 per cent above and a like amount below normal, in order to see the 
influence of this factor upon the performance of the motor. 

Report.—(1) Plot performance curves similar to those shown in Fig. 1. 

(2) If readings have been taken with the field current below normal, and at a different 
line voltage, indicate the results on the same curve sheet, preferably in a different colored ink. 

(3) Answer the following questions: 

(a) What makes a shunt motor slow down when the load increases? 
(b) Why does a shunt motor run faster with a weakened field? 

(c) Why is the efficiency low at a small load? 

(d) Why does the efficiency decrease beyond a certain load? 


kilowatts output = 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 205-2. BRAKE TEST ON A SERIES MOTOR 


Apparatus.—Series-wound motor; Prony brake; ammeter; voltmeter; starting rheostat; 
double-pole switch and fuses for the main circuit (or a circuit breaker); speed counter (or tachom- 
eter); underload circuit breaker (desirable, but not strictly necessary). 

The purpose of the experiment is to investigate the electrical characteristics of a series- 
wound motor similar to those shown in Fig. 1. 

Such characteristics are important for a pros- 
pective user of a motor, in order to determine 
if the motor offered for sale is suitable for 
his purpose. This experiment ought to be 
preceded by experiments E 202-1 ‘‘Prelimin- 
ary study of a direct current machine” and 
EK 203-1 ‘ No-load characteristics of a shunt- 
wound generator” or at least by one of them. 

Connections.—Connect the apparatus as 
shown in Fig. 2 and have all the starting 
resistance in the circuit. Have your connec- 
tions approved by an instructor before start- 
ing the machine. 

Prony Brake.—The simplest device for 
loading motors is a Prony brake, a form of 
which is shown in Fig. 3, in side view and 
in cross-section. It consists of an iron band, 
ab, lined with soft wood or with heavy canvas. 
The band embraces the pulley of the motor, and is fastened to a beam, the other end of which 
rests on a scale. 

When the motor revolves, friction is developed between the lining of the brake and the pulley; 


4 load 
load _ 


1% 


3 
vo 
1?) 
Q 
ao 
o 

oo 
3 
oO 
| 
=) 
# 
§ 
5 
i 
a 

= 






Fic. 1.—Electrical Characteristics of a Series Motor. 





Cooling Water 


Starting Rheostat 
Fia. 2.—Connections for the Test. Fig 3.—Prony Brake. 


the power of the motor is thus converted into heat. The brake pressure is regulated by a hand- 
wheel, and in this way any desired load is obtained. The turning moment, or the torque, as it is 
called, is measured on the scale. ae ar 

In most cases it is necessary to carry away the heat developed by friction, in order to pre- 
vent burning of the brake lining. The pulley, shown in the sketch, is cooled by a stream of water 
from the pipe w (see cross-section to the left); the water is thrown by centrifugal force against the 
inner surface of the face of the pulley and the flange prevents it from being spilled. 

If P is the net pressure in pounds on the scale at the end of a lever / feet long, the pressure 


Copyright, 1913, by V. Karapretorr, Published by JouN Witey & Sons, Inc. 


at the end of a lever one foot long is Pl lbs.; consequently the work in foot-pounds, porformed 
during one minute, is P! X2xn, where n is the number of revolutions of the motor per minute. The 
same in horse-power is 
PlLX2rn 
33000 ’ 
or 


Pin 


horse-power = 5059" 

The net pressure is obtained by subtracting from the actual scale reading the pressure exerted 
upon the scale when the motor is stationary with the power off. This initial pressure simply 
represents the unbalanced weight of the brake. Move the arm up and down to eliminate the 
friction in the bearings of the motor when obtaining this initial pressure. 

In the metric system, if P is in kilograms and / in meters, the formula is 


‘ilo Seta eee at PIX2rn/60X9.81__ Pln 
silowatts output =————F o9g_~—Ss«T 973 

Starting the Machine.—Start the machine slowly in order to see if it runs in the proper 
direction for the Prony brake. If not, reverse either the field or the armature terminals; look 
out for the beam of the brake that it does not strike you. 

An Important Precaution.—The student must be very careful not to let the motor run away. 
With a shunt motor the brake can safely be released, since the speed of the motor is practically 
the same at no load as when loaded. In a series motor the speed increases enormously as soon 
as the load is taken off, and either the armature, the commutator, or the bearings are sure to be 
damaged, if the motor be allowed to run at this speed. For this reason, always open the circuat 
before releasing the brake; or at least have a sufficient resistance inserted into the circuit, to keep 
down the speed. 

As an additional precaution against the motor running away, an underload circuit breaker 
may be connected into the circuit; when the load is taken off, the current falls below a certain 
limit, and this device automatically opens the circuit. The student should not, however, rely 
absolutely on this circuit breaker. It may “ stick” just when it is necessary for it to act. It 
is best to have one man of the section stand near the main switch, and open the circuit if the 
motor should reach a dangerous speed. 

Data Sheet.—Record your readings in a data sheet similar to the one shown below. The 
column for field amperes is necessary only if a run is made with the field current weakened. 


Arm. Amps. Field Amps. Volts. Revs. per min. To 





Inst. No. 


Const. 

















Readings.—Before starting the motor, determine the initial pressure of the brake arm upon 
the scale, as explained above. Begin the test at the highest value of the current which the motor 
can safely carry. Read amperes, volts, speed and the brake pressure. Reduce the load in 
approximately equal steps until the safe limit of the motor speed is reached. . At least ten steps 
ought to be taken in order to obtain satisfactory performance curves. 

The performance characteristics of a series-wound motor may be varied within certain limits 
by shunting part of the current around the field winding. If circumstances permit, put a high- 


resistance rheostat around the field winding of the motor and connect a second ammeter to 
read the field current. Adjust the rheostat so that from 80 to 90 per cent of the total current 
flows through the field winding. Take a few readings for the performance curves of the motor 
so adjusted. 
Report.—(1) Plot performance curves similar to those shown in Fig. 1. 
(2) If readings have been taken with the field current below normal, indicate the results 
on the same curve sheet, preferably in a different colored ink. 
(8) Answer the iilgnine questions: 
(a) What makes a series motor run away when the load is removed? 
(b) Why is the series motor used for electric traction and the shunt motor for shop 
drive? 
(c) Explain the shape of the curves with the field weakened as compared to those 
with the full field. 
(d) Can the direction of rotation of a series motor be reversed by reversing the line 
terminals? 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 206-2. LOAD TESTS ON A TRANSFORMER 


Apparatus.—Transformer; two ammeters; two voltmeters; wattmeter; load rheostat; react- 
ance coil (load); switch and fuses. 


Note.—Instead of two voltmeters, a voltmeter connected to a double-throw switch may be used. 
An ammeter may also be transferred between the primary and the secondary circuits by using short- 
circuiting switches (see diagram). 


The purpose of the experiment is to determine the values of the secondary terminal voltage 
when the primary voltage is kept constant and the load is varied. At no load the ratio of the 
voltages, primary and secondary, is very nearly equal to that of the respective number of turns; 
but when currents flow through the transformer windings, these currents cause an ohmic drop 


Voltmeter 


O O 
Double-throw Switch 














— 


Transformer 
; 3 


Secondary 


Source 
f Circuit 


Primary 
Power Circuit 


Double-throw Switch 
© O 


Ammeter 


Connections for the Test. 


and an inductive drop within the transformer. The result is that the secondary terminal voltage 
is lower than that at no load. This circumstance is of importance when incandescent lamps 
are connected to the transformer, because the voltage across the lamps depends on the number 
of lamps burning. In the specifications on the delivery of a transformer, the fluctuations between 
the no-load voltage and the full-load voltage are usually limited to a certain percentage, depend- 
ing upon the service for which the transformer is intended. The purpose of this experiment 
is to show the student the order of magnitude of voltage drop and also its dependence upon the 
load amperes and the power factor. With lagging currents, the lower the power factor the 
lower is the secondary terminal voltage. | 

Connections.—The connections are as shown in the diagram, provided that only one volt- 
meter and one ammeter are used. The switches K; and K2 must be closed while transferring the 
ammeter, and one of them must be opened when taking a reading. If two ammeters are used 
they are connected in place of the switches K; and Ke. The reactance, L, is shown in parallel 
with the resistance. &, but the two may also be connected in series. 


Copyright, 1913, by V. Karapretorr. Published by Jonn Witey & Sons, Ive. (OVER) 


Data Sheet.—The data sheet may be similar to the one shown below. 





NON-INDUCTIVE LOAD. INDUCTIVE LOAD....AMPS. CONST. 



















Power 
Factor. 


Sec. 
Amps. 


Prim. 
Volts. 


Sec. Prim. 
Watts. Amps. 


Sec. Sec. 
Amps. Volts. 


Prim. Prim. 


Amps. Volts. Volts. Watts. 














Sec. | Sec. 


Inst. No. 











Readings.—1. Apply a certain load, say 25 per cent overload, entirely non-inductive, and 
gradually decrease it, at the same time introducing more and more of an inductive load,—say 
a choke coil in parallel with the rheostat. Regulate the load so as to keep total amperes constant, 
and observe the variation of the secondary voltage as the power factor decreases. The same 
test can be repeated for 100 per cent load, 75 per cent load, etc. 

2. Now again take a certain non-inductive load and gradually add to it some inductive load, 
keeping the total watts constant. Observe the regulation under these conditions. In this case 
it may be better to begin with the lowest power factor available (largest current), as otherwise the 
ammeter and wattmeter may be overloaded and damaged. Several curves should be taken, for 
various values of watts. 

3. Determine the ratio of voltages at no load, if this has not been done in a. preceding 
experiment. 

The student must keep in mind that voltages must be read to the best possible accuracy, 
because the difference between the no-load voltage and the full-load voltage is only a few per cent 
and this difference may be entirely lost or reversed with careless readings. It is only fair to state 
that in practice the voltage drop in a transformer is hardly ever determined by the direct method 
outlined in this experiment, but usually from a short-circuit test.* 

Report.—1. Give the actual connections used during the experiment. 

2. Plot to the power factor values as abscisse, curves of the volts for constant amperes. Plot 
on the same sheet, curves of the corresponding watts. 

3. Plot similar curves for the tests in which the watts were kept constant, showing also the 
corresponding amperes. 


Note 1.—Per cent voltage drop in a transformer is usually small, therefore, in plotting curves, 
the student is advised to use a suppressed scale for voltages. For instance, in a 110-volt transformer 
it is sufficient to mark 100 at the origin and then mark the divisions 105 and 110. 

Note 2.—The inductive drop in a transformer depends upon the arrangement of the primary 
and the secondary coils. The more the coils are interposed (or sandwiched in), the more the 
leakage flux is broken up and the inductive drop reduced. If a transformer is available in which the 
connections and the arrangement of the coils may be varied at will, it is advisable to take two sets 
of readings, one set with the coils interposed as much as possible and the other set with all the 
primary coils on one side of the core and all the secondary coils on the other side. It will be found 
that the voltage drop is several times higher in the second case. 


* See V. Karapetoff, “ Experimental Electrical Engineering,” Vol. II, Arts. 498 to 504. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 206-2. LOAD TESTS ON A TRANSFORMER 


Apparatus.—Transformer; two ammeters; two voltmeters; wattmeter; load rheostat; react- 
ance coil (load); switch and fuses. 


Note.—Instead of two voltmeters, a voltmeter connected to a double-throw switch may be used. 
An ammeter may also be transferred between the primary and the secondary circuits by using short- 
circuiting switches (see diagram). 


The purpose of the experiment is to determine the values of the secondary terminal voltage 
when the primary voltage is kept constant and the load is varied. At no load the ratio of the 
voltages, primary and secondary, is very nearly equal to that of the respective number of turns; 
but when currents flow through the transformer windings, these currents cause an ohmic drop 


Voltmeter 







O O 
Double-throw Switch 









-—Ff 
i 1 | 
ae ) 
— d 
= rt 
q 







Source 
of 
Power 


Secondary 


Primary Circuit 


Circuit 


Double-throw Switch 
O O 


Ammeter 


Connections for the Test. 


and an inductive drop within the transformer. The result is that the secondary terminal voltage 
is lower than that at no load. This circumstance is of importance when incandescent lamps 
are connected to the transformer, because the voltage across the lamps depends on the number 
of lamps burning. In the specifications on the delivery of a transformer, the fluctuations between 
the no-load voltage and the full-load voltage are usually limited to a certain percentage, depend- 
ing upon the service for which the transformer is intended. The purpose of this experiment 
is to show the student the order of magnitude of voltage drop and also its dependence upon the 
load amperes and the power factor. With lagging currents, the lower the power factor the 
lower is the secondary terminal voltage. _ 

Connections.—The connections are as shown in the diagram, provided that only one volt- 
meter and one ammeter are used. The switches Ki and Kez must be closed while transferring the 
ammeter, and one of them must be opened when taking a reading. If two ammeters are used 
they are connected in place of the switches K; and Ke. The reactance, L, is shown in parallel 
with the resistance. 2, but the two may also be connected in series. 


Copyright, 1913, by V. ISaARapetorr. Published by Joun Winey & Sons, Inc. . (OVER) 


Data Sheet.—The data sheet may be similar to the one shown below. 


NON-INDUCTIVE LOAD. INDUCTIVE LOAD... -AMPS. CONST. 

















Sec. 
Amps. 


Power 
Factor. 


Prim. 
Volts. 


Sec. Prim. 
Watts. Amps. 


Sec. 


Prim. Prim. Sec. 
Amps. Volts. 


Amps. Volts. Volts. Watts. 














Sec. | Sec. 








| | 








Readings.—1. Apply a certain load, say 25 per cent overload, entirely non-inductive, and 
gradually decrease it, at the same time introducing more and more of an inductive load,—say 
a choke coil in parallel with the rheostat. Regulate the load so as to keep total amperes constant, 
and observe the variation of the secondary voltage as the power factor decreases. The same 
test can be repeated for 100 per cent load, 75 per cent load, ete. ° 

2. Now again take a certain non-inductive load and gradually add to it some inductive load, 
keeping the total watts constant. Observe the regulation under these conditions. In this case 
it may be better to begin with the lowest power factor available (largest current), as otherwise the 
ammeter and wattmeter may be overloaded and damaged. Several curves should be taken, for 
various values of watts. 

3. Determine the ratio of voltages at no load, if this has not been done in a preceding 
experiment. 

The student must keep in mind that voltages must be read to the best possible accuracy, 
because the difference between the no-load voltage and the full-load voltage is only a few per cent 
and this difference may be entirely lost or reversed with careless readings. It is only fair to state 
that in practice the voltage drop in a transformer is hardly ever determined by the direct method 
outlined in this experiment, but usually from a short-circuit test.* : 

Report.—1l. Give the actual connections used during the experiment. 

2. Plot to the power factor values as abscissa, curves of the volts for constant amperes. Plot 
on the same sheet, curves of the corresponding watts. 

3. Plot SS AulAg curves for the tests in which the watts were kept constant, showing also ths 
corresponding amperes. 


Note 1.—Per cent voltage drop in a transformer is usually small, therefore, in plotting curves, 
the student is advised to use a suppressed scale for voltages. For instance, in a 110-volt transformer 
it is sufficient to mark 100 at the origin and then mark the divisions 105 and 110. 

Note 2.—The inductive drop in a transformer depends upon the arrangement of the primary 
and the secondary coils. The more the coils are interposed (or sandwiched in), the more the 
leakage flux is broken up and the inductive drop reduced. If a transformer is available in which the 
connections and the arrangement of the coils may be varied at will, it is advisable to take two sets 
of readings, one set with the coils interposed as much as possible and the other set with all the 
primary coils on one side of the core and all the secondary coils on the other side. It will be found 
that the voltage drop is several times higher in the second case. 


*See V. Karapetoff, “ Experimental Electrical Engineering,” Vol. II, Arts. 498 to 504. 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 207-1. NO-LOAD CHARACTERISTICS OF AN ALTERNATOR 


Apparatus.—The alternator with a suitable drive; voltmeter; field rheostat; switch and 
fuses; speed counter (or tachometer). 

The purpose of the experiment is to investigate the dependence of the voltage induced 
in the armature conductors upon the exciting current and the speed. When the field current 
is kept constant (constant flux) and the speed of rotation is varied, the induced voltage is directly 
proportional to the number of revolutions per minute, according to the fundamental law of induc- 
tion. At a constant speed the induced voltage is at first proportional to the field current, but 
then increases more and more slowly, due to saturation in the iron parts of the magnetic circuit. 
A curve which gives the relation between the field current and the induced voltage at a constant 
speed is called the no-load characteristic of the alternator. 

Connections.—The connections for a single-phase machine are shown in the diagram. If a 
three-phase machine, only, is available, it is sufficient to 
take readings on one phase, having satisfied oneself that 
the three induced voltages are substantially equal. 

Data Sheet.—The data sheet must have columns for 
the field amperes, the induced volts and the speed. 

Order of the Experiment.—1. Make rough sketches of 
the machine, especially its longitudinal and transverse cross- 
sections. Make detailed sketches of parts which do not Cofncationa for the’ Test. 
show well in these two cross-sections. 

2. Run the machine at its rated speed and increase the field excitation in steps; read the 
corresponding alternating voltages and field amperes. Take, also, readings with decreasing field 
current so as to investigate the influence of residual magnetism. 

3. Having opened the field circuit, raise the speed of the machine to its highest safe limit 
and then excite the machine to the highest feasible value of voltage, determined for instance by 
the highest reading possible on the voltmeter. Keep the field current constant and reduce the 
speed to zero, in steps. Read the induced volts and the revolutions per minute at each step. 

4, Make a similar test at a different value of the field current. 

Report.—1. Draw neat sketches of the machine, approximately to scale, and give a concise 
description of the principal parts. If familiar with shop practice, indicate the principal operations 
necessary for manufacturing these parts. 

2. Plot the no-load characteristic at the rated speed and show how to calculate the frequency 
in cycles per second and in alternations per minute. 

3. Prove from your readings that, at a constant field current, the induced voltage is pro- 
portional to the speed. 


4, Answer the following questions: 
(a) Why is a direct-current machine usually self-excited while an alternator is not? 


(b) What is the reason that, in direct-current machines, the armature is always the 
revolving part while the field is stationary, and that, in alternators, the 
opposite is usually the case. 





Copyright, 1913, by V. Karaprrorr, Published by Joun Winey & Sons, Ine. 





THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 207-2. VOLTAGE CHARACTERISTICS OF A LOADED ALTERNATOR 


Apparatus.—Alternator with a suitable drive; ammeter; voltmeter; field rheostat; load 
rheostat; choke-coil (load); wattmeter; switch and fuses; speed counter (or tachometer). 

The purpose of the experiment is: (a) To investigate the influence of the load of an alternator 
upon its terminal voltage when the field current is kept constant; and (6) to find out the increase 
in the field current required with different loads in order to keep the terminal voltage constant. In 
other words, this experiment corresponds to experiments E 203-2, ‘‘ Voltage Characteristics of a 
Shunt-wound Generator” and E 203-38, ‘‘ Excitation Characteristics of a Shunt-wound Generator.”’ 

In a direct current machine the load is given in amperes, while in an alternator the load is fully 
determined by the amperes and the power factor. In specifications on the delivery of an alter- 
nator, the permissible voltage fluctuation between no load and full load is usually expressed by 
requiring a certain per cent voltage regulation. 

Voltage regulation is defined in this country as the expression 


Eo- sk 
NE A = eras E, 


where Eo is the no-load voltage of the machine and £ is the rated terminal voltage at the rated 
load. It is assumed that the field current and the speed remain the same between no load 
and full load. For instance, if the rated voltage of the machine is 2200, and the voltage rises to 
2420 when the load is thrown off, the voltage regulation is 


100(2420 — 2200) /2200 = 10 per cent. 


Connections.—The connections for a single-phase alternator are shown in Fig. 1. If a three- 
phase machine, only, is available, the student 
is advised to wire up and load one phase only, 
so as to simplify readings. The relations 
are qualitatively the same when all the three 
phases are loaded. While three ammeters 
are shown in the diagram, it is preferable 
to use only one ammeter and to transfer it 
to the three branches by means of a suitable 
plug-board or switches. Such a plug-board 
must preferably be so arranged that before a plug connected to the ammeter is completely 
withdrawn a brass spring closes the circuit, short-circuiting the place of the ammeter. 

Data Sheet.—A sample data sheet is shown below. The speed must be kept as nearly 
constant as possible, but unavoidable variations must be recorded. 





Fra. 1.—Connections for a Single-phase Alternator. 








Load Amps. Constant at per cent of Full Load Rating. 








Amperes. 
Volts. Watts. 





Speed. 
Non-Ind., Ind. Total. 














Copyright, 1913, by V. Karaprtorr. Published by Joun Witey & Sons, Inc. (OVER) 


Readings.—1. Load the machine at its rated voltage, about 25 per cent overload in rated 
current, and at the lowest power factor obtainable. Read the quantities indicated in the data 
sheet. Keep the field current and the total load amperes constant; reduce the inductive amperes 
and increase the non-inductive amperes so as to increase the power factor of the load. Repeat 
the readings at each step. Continue this process until all of the inductance is cut out and the 
load is practically non-inductive. At least eight points are necessary in order to obtain a 
satisfactory curve. 

2. Repeat the same test with the rated current and, if time permits, also at 75 per cent and 
at 50 per cent of the rating. It must be understood that the field current is kept constant during 
each set of readings, but that it is different for the different values of the total load current. 

3. Adjust the load as under (1); keep the terminal voltage and the-load amperes constant, 
gradually reducing the field current as the power factor of the load increases. 

4, Repeat test (3) at the rated current and, if time permits, at 75 per cent and at 50 per 
cent of the rated current. 

5. Take the no-load saturation curve of the machine if one has not been previously obtained 
(Exp. E 207-1). 

Report.—1. Plot curves of terminal volts against per cent power factor when the field 
current is kept constant. Mark a horizontal straight line indicating the no-load voltage obtain- 
able with the same field current as at the rated load and 100 per cent power factor. This voltage 
is taken from the no-load saturation curve. 

2. Plot values of field current against per cent power factor as abscissee from the readings 
when the terminal voltage was kept constant. Mark on the same curve sheet a horizontal straight 
line indicating the field amperes necessary for the rated voltage at no load. This value is taken 
from the no-load saturation curve. 

3. Plot the no-load saturation curve if it has not been plotted in a preceding experiment. 

4. Calculate the per cent voltage regulation at the rated current and a non-inductive load; 
also for the rated current and an inductive load at the lowest power factor obtainable. 

5. If you are familiar with vector diagrams, select a few sets of readings of total amperes and 
amperes through the resistance and the reactance; 
construct triangles of currents and check the values 
of the power factor obtained from the wattmeter 
readings with those calculated from triangular cur- 
rents. In Fig. 2 the vectors Ai, Az, and A3 represent 
the three ammeter readings corresponding to Fig. 1. 
E is the vector of the line voltage. The cosine of 
the angle marked ¢ represents the power factor; its 
value must check with the value obtained by dividing the true power as read on the wattmeter 
by the apparent power which is the product of the terminal voltage times the total load 
amperes. 

6. Look up the theory of the armature reaction in an alternator and explain the reason 
why the voltage regulation is poorer at the lower values of the power factor. 





Fic. 2.—Vector Diagram. 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 208-1. STARTING AN INDUCTION MOTOR 


Apparatus.—Three-phase induction motor; starting transformers; two ammeters; volt 
meter; Prony brake; stop-watch; three-phase circuit breaker (or switches and fuses). 

The purpose of the experiment is to learn the wiring of a three-phase induction motor, to 
acquire fluency and precision in starting and stopping an induction motor and to investigate the 
effect of the starting voltage upon the starting torque. In this country, induction motors 
with a short-circuited or squirrel cage secondary are used almost exclusively, especially in small 
and medium sizes, and, for this reason, this exercise is limited to motors of this type. To avoid 
an objectionable rush of current during the first few instants of starting, an induction motor 
is usually started on a reduced voltage. As soon as the motor has acquired a certain speed it is 
switched over to the full voltage. In many cases, two or more intermediate voltages are used 
in succession in order to avoid a sudden change. The intermediate voltages are obtained by 
means of two auto-transformers connected in V, as shown in the figure. The transformer wind- 
ings are provided with taps; these taps are connected in succession to the motor terminals. 
The lowest permissible value of the starting voltage depends upon the required starting torque; 
the latter decreases approximately as the square of the voltage. 

Connections.—The connections are shown in the accompanying diagram. If a regular motor 
starter is available, it has switches or a controller drum inside 
by means of which the necessary changes from tap to tap are 
made. If only two ordinary auto-transformers are available, 
the student is expected to arrange the switches so as to be 
able to change from fractional voltages to the full voltage. 

Readings.—1. Investigate the behavior of the motor when 
starting at no load with different taps; vary the time during 
which the motor is run on a reduced voltage. The problem 
is to get such a low voltage, such steps, and such intervals of 
time between the steps as to bring the motor up to its full 
speed with the least inrush of current from the line. Having 
the ammeters in the circuit, the student will find the best 
conditions after several trials. Record volts, amperes and seconds of time for each trial. 

2. Make a similar investigation with a definite starting torque, for instance, one quarter 
of the full-load torque. A load torque may be obtained to a sufficient degree of accuracy by 
means of a Prony brake. For a description of the latter and the necessary computations, see 
Experiments E 205-1 and E 205-2 on brake tests of direct-current motors. 

3. Repeat the tests with a torque equal to one-half of the full-load torque, and again with 
three-quarters of the full-load torque. It is probable that, at a torque exceeding this value, an 
excessive line current would be required for starting. 

In performing these experiments, the student must be careful not to overheat the motor and 
the starting transformers because the apparatus is usually designed for starting at infrequent 
intervals only. 

Report.—Give the numerical results of your tests and specify the exact steps and the dura- 
tion of each step for the best conditions in starting the motor with various values of the starting 
torque. 





Connections for the Test. 


Copyright 1913. Published by Jonn Witny & Sons, Ine. 


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THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 208-2. LOAD TEST ON AN INDUCTION MOTOR 


Apparatus.—Induction motor with the necessary starting device; ammeter; voltmeter ; watt- 
meter; voltmeter switch; ammeter transfer switch; speed counter; generator to be used as a 
load; load rheostat for the same; slip meter (desirable but not necessary). 

The purpose of the experiment is to obtain performance curves similar to those shown in 
Fig. 1. Such curves are important for a perspective 
user of a motor, in order to determine if a motor 
offered for sale is suitable for his purpose. This 
experiment ought to be preceded by Experiment 
f 20 -1, “ Starting an Induction Motor.” The curve 
marked “ True H.P. Input” is obtained by dividing 
the watts input to the motor by 746. Similarly, 
the ordinates of the curve of “ Apparent H.P. Input ” 
represent the volt-ampere input divided by 746. The 
ordinates of the efficiency curve are equal to the 
ratio of the ordinates of the output and the true input 
curves. The ordinates of the power-factor curve 
represent the ratio of true input to apparent input. 

The output of the motor may be measured 
with a Prony brake in a manner similar to that a. 1—Performance Curves cf an Induction 
described in Experiments E 205-1 and E 205-2 on Motor. 
brake tests of direct-current motors. However, in 
this case the student is advised to calculate the output from the input into the motor and 
the losses in the motor. This method is preferable because experience shows that it is rather 
difficult to keep a load constant for any length of time by means of a Prony brake. In the 
case of a direct-current motor the readings are comparatively simple and can be made in a few 
seconds. With an induction motor the time necessary for taking readings is much longer (see 
the data sheet below), and, besides, the motor must be run for at least half a minute or a minute 
in order to determine its slip accurately. A load consisting of an electric generator, a pump, 
a blower, etc., may be kept steady for any length of time without much difficulty. Incidentally, 
this method has the advantage of giving the student some insight into the theory of the induction 
motor. 

Calculation of the Output from the Input and the Losses.—The losses in an induction motor 
consist of the copper loss in the primary and in the secondary windings, and of the no-load 
losses (iron loss and friction). The latter can be assumed to be practically constant at all loads 
and as having the same value as at no load (hence the name “ no-load losses”’). The primary 
copper loss can be calculated easily if the primary current and the resistance of the primary 
winding are known. The secondary copper loss is proportional to the per cent slip, as is shown 
in the general theory of the induction motor. 

Suppose, first, the resistance of the secondary winding to be zero; the slip would be prac- 
tically zero at any load because the secondary currents necessary for producing a torque could 
be induced with an infinitesimal difference in speed between the revolving flux and the rotor. If, 
however, the secondary winding has an appreciable resistance, a certain finite difference in speed 
is required in order to induce the same torque-producing currents. Thus, the necessary torque 
is obtained at a sacrifice of a certain per cent of speed. The corresponding loss of output is con- 
verted into the /*? heat in the secondary winding. 

The details of the calculation of the output of the motor from its input and the losses may 
be shown best in a numerical example. A 10 horse-power, 440-volt, 3-phase induction motor 


Synchr.Speed 











————— > HP. Output 


Copyright, 1913, by V. KaraPErorr. Published by Jonn Winey & Sons, Inc. 


was found to take 800 watts at no load; the no-load current was 5.5. amperes per phase, primary 
resistance .075 ohm per phase. From these data the iron loss and friction should first be com- 
puted; these are equal to 800 watts less a correction for the copper losses in the primary and the 
secondary windings. But the secondary copper loss is negligible at no load, because the slip is 
very small. Thus, the correction amounts to 3X65. 5.52X0.75=68 watts, aod the losses in ques- 
tion are equal to 


800 —68 = 732 watts. 


Take now a point on the load curve, for instance, corresponding to an input of 15 amperes 
per phase. Suppose that the power reading at this input was 9920 watts, and the slip 5.4 per 
cent. The primary copper loss is 


3X 1520.75 =506 watts. 

Therefore, 
output+sec. copper loss = 9920— (506-+732) =8682 watts, the last number representing the input 
into the secondary. 

The secondary copper loss constitutes a part of the input into the secondary, proportional 
to the per cent slip. Thus, in our case, 

sec. copper loss = 86825 = 469 watts. 
Therefore, 
output = 8682 — 469 = 8213 watts=11 horse-power. 


Knowing the output corresponding to a given input, the efficiency, the torque, and the other 
quantities shown in Fig. 1 may easily be calculated. 

Measuring the Input.—The electrical power input to a three-phase motor is usually measured 
by the so-called two-wattmeter method, as shown in Fig. 2. One of the line wires, say B, is assumed 
to be a common return wire for two other wires. The 
power is measured between the wires A and B, and then os 
between C and B, and the wattmeter readings are added @ 
together in order to get the total input to the motor. UM , Onm 
Accordingly, the series winding of one wattmeter is con- 
nected into the line A, and its shunt winding across A-—B. 
The series winding cof the other wattmeter is connected 
into the line C, and the potential winding across C—B. Fia. 2.—Connections for the Test. » 

With proper transfer switches only one wattmeter is 
needed; it is connected in succession in the two positions shown in Fig. 2, and the readings are 
added algebraically. 

Theory and experience show that this method gives correct results for the total power input 
on balanced as well as on unbalanced loads. However, the two component readings are equal 
only when the load is balanced and non-inductive (power factor of 100 per cent). In an induc- 
tion motor:the power factor is always less than 100 per cent, so that one of the wattmeter readings 
is always smaller than the other. On light loads, when the power factor of an induction motor 
becomes less than 50 per cent, one of the wattmeters begins to give negative deflections. In this 
case, reverse the potential or the series leads of the meter, and take the difference of the two 
readings instead of their sum. 

For this reason it is advisable to begin the test at the maximum load (say 25 per cent over- 
load) where the power factor is surely higher than 50 per cent, and then reduce the load by steps 
to zero. Then one cannot miss the point at which it becomes necessary to reverse the leads of 
one of the wattmeters. If a polyphase wattmeter is available, total power is obtained from 
one reading. 

It would hardly be practicable to provide separate ammeters and voltmeters for each phase; 
a special “polyphase board ” or a system of transfer switches is used by means of which the same 
instruments are connected in succession in the three phases. 


Ammeter 








Voltmeter 





a 


Measuring Frequency and Speed.—The method of measuring the speed of an induction 
motor deserves special attention. The speed depends on the load and on the frequency of the 
supply currents, as the latter determine the speed of the revolving field. If the power for test- 
ing is taken from a commercial supply, the frequency may at times be several per cent above or 
below the normal, and unless the exact frequency is known at the time when the speed is taken, 
the speed determination is of little value. 

When the generator is accessible, its speed may be measured sumultaneously with that 
of the motor, so that the two speeds refer to the same frequency. If the generator is not accessible, 
a small synchronous motor may be run from the same line to which the induction motor is con- 
nected. As the speed of the synchronous motor is always equal to that of the generator and 
automatically follows the variations in speed of the latter, the synchronous motor will give at 
any moment the actual speed of the generator. Special instruments, so-called frequency meters, 
may also be used for measuring the frequency of the supply. 

Instead of measuring the speed of the motor and the frequency of the supply, it is preferable 
to measure simultaneously the speed and the slip of the induction motor; their sum gives the 
synchronous speed, and consequently the frequency of the supplied currents. If, for instance, 
the motor speed is 702 r.p.m., and the slip 22 r.p.m., the synchronous speed is 724 r.p.m. If the 
motor is a 10-pole machine, the frequency of the supply is at that particular moment 7240 
alternations per minute, instead of the standard 7200. In plotting the speed curve, the cor- 
responding correction must be made. 

Slip can conveniently be measured by devices called slip meters. The three principal types 
of these devices are based on the following principles: stroboscopic, vibrating reed, and rectifica- 
tion of alternating currents. For a detailed description of these devices see V. Karapetoff, Exper- 
imental Electrical Engineering, Vol. I, Arts. 340 and 341. 

Data Sheet.—The readings may conveniently be recorded in a data sheet similar to the one 
shown below. If the output is calculated from the losses and no brake is used, the last column 
is omitted. 








Amperes. Volts. Watts. Speed. Torque. 
Slip 
Indic. 


CA A—AB C—CB Motor. Synchr. Lbs. 






































































Readings.—1. Wire up the motor as shown in Fig. 2, and, if possible, run it light for at least 
half an hour in order to obtain a steady condition of the lubrication of the bearings. 

2. Begin the test with the heaviest load possible, say about twenty-five per cent above the 
rating of the motor. Read the quantities indicated in the data sheet, transferring the ammeter, 
the voltmeter and the wattmeter from phase to phase by means of switches. The readings 
must be taken very accurately in order to obtain satisfactory curves. 

3. Reduce the load in steps and at each step repeat the readings. From eight to ten points 
are desired on the curves. 

4. Take careful readings of amperes and watts at no load. 

5. Measure accurately the resistances of the primary windings by the drop-of-potential method, 
using direct current. Measure by thermometers the temperature of the windings while deter- 
mining their resistance. 


Report.—Calculate the data and plot curves as indicated in Fig. 1. In figuring ‘out the 
primary copper loss, use the value of the primary resistance at a temperature of 50° C. above 
the usual temperature of the room. 

It should be borne in mind that, when a resistance is measured between two terminals of 
a Y-connected induction motor, the result thus obtained represents the double resistance per 
phase. Therefore, in order to obtain the average resistance per phase, take the resistance between 
the terminals A—~B, A—-C, and B-C, add them together, and divide the result by six. 

Plot on a separate sheet, curves of total losses, primary and secondary J?R losses, and iron 
loss+friction, against horse-power output as abscisse, so as to have a clear expression of the 
relative importance of these losses at various loads. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 209-1. CHARGING A STORAGE BATTERY IN SECTIONS 


Apparatus.—A storage battery which may be divided into two or three sections at will; double- 
pole, double-throw switch; regulating rheostat; two ammeters;* voltmeter; fuses (or circuit 
breaker). 

The purpose of the experiment is to learn methods of charging batteries in sections, with- 
out the use of a booster, end-cell switches or other refinements. The methods described below 
are sufficient for charging small batteries. The experiment ought to be preceded by Experiment 
E 14-2 on the charge and discharge of a single cell. 

The simplest method for charging and regulating a storage battery is shown in the diagram 
below. The battery is divided into two halves which are connected in series for discharging, and 
in parallel for charging. This is done in order to secure a sufficient voltage for charging, without 
affecting the line voltage maintained by the generator. An example will make this clearer. 
Consider a battery intended for an ordinary 110-volt lighting circuit; the voltage of each cell 


Load 





Connections for the Experiment. 


at the end of discharge is about 1.8 volts, therefore the number of cells required is 110+-1.8 =62. 
But the voltage necessary with this number of cells at the end of a charge is =2.6X62=161 volts, 
which is far above the line voltage. With the battery divided into two halves in parallel, only 
80.5 volts are required for charge; the excess voltage of the line is taken up by the rheostat R. 
The battery output on discharge is also regulated by this rheostat. This method, although very 
simple, is used only in small installations, or where the loss of power in the rheostat is not 
objectionable. 

A more economical method is to divide the battery into three equal parts; let them be denoted 
A, Band C. The parts A and B are first charged in series for one-half of the time necessary for 
full charge, then B and C are charged in series for one-half of the time, and finally C and A for 
one-half of the time. Less energy is wasted in the resistances with this arrangement, although 
it takes longer to charge the battery. The voltage at the end of the charge is 7X161=107 
volts. 

Other combinations are also possible; for instance, A and B may be connected in parallel 
with each other and in series with C. The set is charged at the full rate until C is completely 
charged. Then C is disconnected, A and B are connected in series, and the charge is completed. 

Order of the Experiment.—Wire up the two halves of a battery, as shown in the diagram, 
and make connections to a suitable generator. Provide a load in the form of adjustable resistances, 
and operate the installation under the following conditions: 

(a) Both the battery and the generator supplying power to the line. 

(b) The battery being charged, the generator at the same time supplying power to the line. 

(c) The battery alone supplying power, the generator shut down. 

@ The generator working alone, the battery being disconnected for inspection a repairs. 


* One of them preferably with zero in the center of the scale. 


Copyright, 1913, by V. Karaprrtorr, Published by Joun Witey & Sons, Ine, ‘ (OVER) 


For each of these conditions select a few characteristic loads (light load, medium load, full 
load and overload) and take all the necessary ammeter and voltmeter readings, so as to have a 
complete record of the electrical relations in the circuit with special reference to the performance 
of the battery. Observe the voltage and current fluctuations, when the load is varied, first 
gradually and then suddenly. 

Devise a convenient arrangement of switches for charging the battery in three parts, as 
explained in the preceding article. Connect the battery accordingly and observe the process of 
charging. 

Report.—1. Draw the exact diagrams of the connections used during the experiment. 

2. Give your data in regard to the. operation of the installation under conditions (a) to (d). 

3. Compare critically the three methods for charging the battery in sections and give a few 
rough calculations as to the comparative loss of energy in the regulating rheostat. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 210-2. INFLUENCE OF THE TRANSMISSION VOLTAGE AND OF THE 
CROSS-SECTION OF THE LINE ON ITS REGULATION 


Apparatus.—Artificial single-phase transmission line; load rheostat; load reactance coil; 
ammeter; line voltmeter; low-reading voltmeter; wattmeter; double-throw voltmeter switch; 
main switch and fuses (or a circuit breaker). 


Note.—If the experiment is to be performed with direct-current, omit the reactance coil and the 
wattmeter. 


The purpose of the experiment is to illustrate the influence of some of the factors which 
determine the size of the conductor used in a long-distance transmission line. Some other 
factors are discussed in the companion leaflet EK 210-1 “ Influence of Load and of Distance of 
Transmission on Voltage Regulation of a Line.” 

With a given generator voltage the voltage at the receiver end is determined by the voltage 
drop in the line. But this voltage drop depends upon the following factors: (a) the resistance 
of the line conductors per mile; (b) the reactance of the line conductors per mile; (c) the capacity 
of the line (which is neglected in these experiments); (d) the length of the line; (e) the magnitude 
of the load; (f) the power factor of the load. In order to give a satisfactory service the voltage 
drop must not exceed a certain percentage of the normal receiver voltage; otherwise, individual 
customers would be annoyed by the voltage fluctuations at the terminals of their lamps and 
motors caused by the load fluctuations of other customers. In this country per cent voltage 
regulation is defined as 

Eo-Fi 
100 me 
where E> is the receiver voltage at no load, and EF, is the receiver voltage at the rated load, the 
generator voltage remaining constant. 

When the load on a given line has grown to such a magnitude that the voltage drop is too large 

for a satisfactory service, there are usually two 





ways out of the difficulty; either to increase the 2 
cross-section of the conductors or to raise the trans- : 3 
mission voltage. The purpose of this experiment o[24 
is to study the influence of these two factors 3] 2 
(cross-section of the conductor and the transmis- & 3 
sion voltage) upon voltage drop and regulation. § S 
Connections.—The diagram of connections is 5 


shown in the figure. It is not necessary to have 
a transmission line strung in the laboratory. All 
that is needed is a resistance and a reactance in 
series with it which to a certain scale represent 

those of an actual line.* Moreover, it is sufficient EovOOOTTIIY 

to have the resistance and the reactance of the Abhi 

two conductors of a single-phase line concentrated 

in one, and to replace the other conductor by an ideal one without resistance or reactance 
(artificial ground). The advantage of such an arrangement is that the whole drop. is con- 
centrated upon one conductor and may be measured directly with the low-reading voltmeter 
D.V. (drop voltmeter). Three separate lines are shown in the diagram, which may differ from 
one another in their electrical characteristics. Any of the three may be used by simply chang- 


* For actual values of resistances and reactances of transmission lines consult tables in various electrical hand- 
books, pocket-books, and treatises on power transmission. 


Copyright, 1913, by V. Karaprrtorr, Published by Joun Witey & Sons, Inc, (OVER) 


ing the position of the plug p. The voltmeter V measures both the generator and the receiver 
voltages by means of the switch S. In a direct-current line the voltage drop in the line is the 
arithmetical difference of the generator and the receiver voltages; but in an alternating-current 
line the three voltages are not in phase, so that the voltage drop is larger than the arithmetical 
difference between the generator and the receiver voltage. 

A variable generator voltage required in this problem may be obtained in one of the following 
ways: (a) by using a separate generator which may be excited at will; (6) by connecting a 
resistance in series with the line, so as to cut down the available voltage; (c) with alternating cur- 
rents the most convenient way is to use a transformer or auto-transformer with several taps on 
the windings. 

Data Sheet.—Record load amperes, watts, and volts; generator volts; and the voltage 
drop in the line. Also note the resistance and the reactance of the line. 

Readings.—1. Begin the experiment with the most unfavorable conditions, that is, a line 
of minimum cross-section possessing a maximum resistance and reactance, the lowest generator 
voltage, the heaviest load, and the lowest power factor. 

2. Repeat the test with a non-inductive load. 

3. Keep the generator voltage and the load the same as in the preceding two tests, and use 
lines of larger cross-section. In adjusting the resistance and’ the reactance, remember that 
when the cross-section of a conductor is doubled its resistance is reduced to one-half, but the 
reactance of the line, with the same spacing, may be reduced only a few per cent. 

4, Use the same watts load and the same line as in the first two experiments, but raise the 
generator voltage in steps, taking readings at each step. 

Report.—1. Plot curves or tabulate your results showing how the regulation is improved 
and the line drop reduced by using larger size conductors. 

2. Plot similar curves showing the effect of the transmission voltage, at a constant load. 

3. Answer the following questions: 

(a) Why are higher voltages used for longer transmission lines and vice versa? 

(b) What would you do in a given case to improve the regulation of a line, raise the 
transmission voltage or increase the cross-section of the conductors? Mention 
some arguments in favor of and against each solution. 

(c) The conductivity of aluminum is about 62 per cent of that of copper, and its specific 
weight is about 30 per cent of that of copper. If copper costs 15 cents a pound, what must 
be the maximum cost of aluminum per pound in order that an aluminum transmission line be 
at least as cheap as that made of copper? Both lines must have the same resistance per mile. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 210-2. INFLUENCE OF THE TRANSMISSION VOLTAGE AND OF THE 
CROSS-SECTION OF THE LINE ON ITS REGULATION 


Apparatus.—Artificial single-phase transmission line; load rheostat; load reactance coil; 
ammeter; line voltmeter; low-reading voltmeter; wattmeter; double-throw voltmeter switch; 
main switch and fuses (or a circuit breaker). 


_ Note.—If the experiment is to be performed with direct-current, omit the reactance coil and the 
wattmeter. 


The purpose of the experiment is to illustrate the influence of some of the factors which 
determine the size of the conductor used in a long-distance transmission line. Some other 
factors are discussed in the companion leaflet EH 210-1 “‘ Influence of Load and of Distance of 
Transmission on Voltage Regulation of a Line.” 

With a given generator voltage the voltage at the receiver end is determined by the voltage 
drop in the line. But this voltage drop depends upon the following factors: (a) the resistance 
of the line conductors per mile; (b) the reactance of the line conductors per mile; (c) the capacity 
of the line (which is neglected in these experiments); (d) the length of the line; (e) the magnitude 
of the load; (f) the power factor of the load. In order to give a satisfactory service the voltage 
drop must not exceed a certain percentage of the normal receiver voltage; otherwise, individual 
customers would be annoyed by the voltage fluctuations at the terminals of their lamps and 
motors caused by the load fluctuations of other customers. In this country per cent voltage 
regulation is defined as 

Ko— Fi 
100X hale 
where E> is the receiver voltage at no load, and EF; is the receiver voltage at the rated load, the 
generator voltage remaining constant. 

When the load on a given line has grown to such a magnitude that the voltage drop is too large 
for a satisfactory service, there are usually two 
ways out of the difficulty; either to increase the 
cross-section of the conductors or to raise the trans- 
mission voltage. The purpose of this experiment 
is to study the influence of these two factors 
(cross-section of the conductor and the transmis- 
sion voltage) upon voltage drop and regulation. 

Connections.—The diagram of connections is 
shown in the figure. It is not necessary to have 
a transmission line strung in the laboratory. All 
that is needed is a resistance and a reactance in 
series with it which to a certain scale represent 
those of an actual line.* Moreover, it is sufficient 
to have the resistance and the reactance of the Eee bs 
two conductors of a single-phase line concentrated 
in one, and to replace the other conductor by an ideal one without resistance or reactance 
(artificial ground). The advantage of such an arrangement is that the whole drop is con- 
centrated upon one conductor and may be measured directly with the low-reading voltmeter 
D.V. (drop voltmeter). Three separate lines are shown in the diagram, which may differ from 
one another in their electrical characteristics. Any of the three may be used by simply chang- 


Generator Bus Bars 





Artificial Ground 


* For actual values of resistances and reactances of transmission lines consult tables in various electrical hand- 
books, pocket-books, and treatises on power transmission. 


Copyright, 1913, by V. Karaprrorr. Published by Joun Witey & Sons, Inc. (OVER) 


ing the position of the plug p. The voltmeter V measures both the generator and the receiver 
voltages by means of the switch S. In a direct-current line the voltage drop in the line is the 
arithmetical difference of the generator and the receiver voltages; but in an alternating-current 
line the three voltages are not in phase, so that the voltage drop is larger than the arithmetical 
difference between the generator and the receiver voltage. 

A variable generator voltage required in this problem may be obtained in one of the following 
ways: (a) by using a separate generator which may be excited at will; (6) by connecting a 
resistance in series with the line, so as to cut down the available voltage; (c) with alternating cur- 
rents the most convenient way is to use a transformer or auto-transformer with several taps on 
the windings. 

Data Sheet.—Record load amperes, watts, and volts; generator volts; and the voltage 
drop in the line. Also note the resistance and the reactance of the line. 

Readings.—1. Begin the experiment with the most unfavorable conditions, that is, a line 
of minimum cross-section possessing a maximum resistance and reactance, the lowest generator 
voltage, the heaviest load, and the lowest power factor. 

2. Repeat the test with a non-inductive load. 

3. Keep the generator voltage and the load the same as in the preceding two tests, and use 
lines of larger cross-section. In adjusting the resistance and the reactance, remember that 
when the cross-section of a conductor is doubled its resistance is reduced to one-half, but the 
reactance of the line, with the same spacing, may be reduced only a few per cent. 

4, Use the same watts load and the same line as in the first two experiments, but raise the 
generator voltage in steps, taking readings at each step. 

Report.—1. Plot curves or tabulate your results showing how the regulation is improved 
and the line drop reduced by using larger size conductors. 

2. Plot similar curves showing the effect of the transmission voltage, at a constant load. 

3. Answer the following questions: 

(a) Why are higher voltages used for longer transmission lines and vice versa? 

(b) What would you do in a given case to improve the regulation of a line, raise the 
transmission voltage or increase the cross-section of the conductors? Mention 
some arguments in favor of and against each solution. 

(c) ‘The conductivity of aluminum is about 62 per cent of that of copper, and its specific 
weight is about 30 per cent of that of copper. If copper costs 15 cents a pound, what must 
be the maximum cost of aluminum per pound in order that an aluminum transmission line be 
at least as cheap as that made of copper? Both lines must have the same resistance per mile. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 211-1. STARTING SYNCHRONOUS MOTORS 


Apparatus.—Synchronous motor; direct-current motor for bringing it up to speed; main 
ammeter; voltmeter; field ammeter; field rheostat; synchronizing lamps; main switch; fuses 
(or preferably a circuit breaker); field switch and fuses; a power-factor meter is desirable but 
not necessary. 

The purpose of the experiment is to learn the usual methods of starting and synchronizing 
a synchronous motor, and to acquire some skill in so doing. Before a synchronous motor can 
be connected to the line and furnish mechanical power, the line voltage and that induced in the 
motor must fulfil three conditions: (1) They must be nearly equal numerically; (2) they must, 
be of the same frequency; (3) they must be in phase with one another. The process of bringing 
a synchronous machine in phase with another or with the line is called synchronizing (bringing 
in step). 

Action of a Synchronous Motor.—The action of a synchronous motor may be explained as 
follows: Suppose the machine to be single-phase and to be brought up to the required speed 
by some other motor. Assume that at a certain moment the relative position of the pole-pieces 
and of the armature winding is such that the winding attracts the pole-pieces. As the machine 
is supposed to revolve synchronously, the pole-pieces change their position during one alterna- 
tion of the supply current by one pole pitch, so that the north poles come in place of the south 
poles, and vice versa. At the same time the direction of the armature current is reversed, so 
that the mutual force between the two is again attraction and not repulsion. 

Another explanation limited to the case of polyphase synchronous motors is that the poly- 
phase armature winding produces a revolving magnetic field which rotates synchronously in the 
air-gap. The field poles of the machine must revolve at the same speed in order that there be 
a constant attraction between the two magnetic fields; otherwise south poles and north poles 
are brought together in succession and the resultant attractions and repulsions neutralize each 
other. 

Starting Synchronous Motors.—The above explanation of the action of the synchronous 
motor shows that it must be started and brought up to full speed before being capable of carry- 
ing a load. The following means are used for starting synchronous motors: (1) A small induc- 
tion motor, mounted on the same shaft with the synchronous motor, belted, or geared to it. 
(2) If a source of direct current is available, the exciter machine belted or directly connected 
to the motor, is sometimes used for starting. (8) The synchronous motor itself (if polyphase) 
is converted into an induction motor and started as such. 

Synchronizing Lamps.—In order to ascertain when the first of the above-named three con- 
ditions is fulfilled, a voltmeter is used, which is connected in succession across the line and across 
the terminals of the motor. The second condition is 
approximately fulfilled when the machine is running 
at its rated speed. 

In order to ascertain when the third condition 
is fulfilled, incandescent lamps (Fig. 1) are con- 
nected across the switch between the motor and 
the line. As long as the motor is not in phase with 
the line, equalizing currents circulate through it, and 
the lamps a serve both to reduce and to indicate these Sale 
currents. By varying the speed of the machine, it is possible to extinguish the lamps; this will 
show that the machine is in perfect synchronism, and the switch can be closed. The sketch 
illustrates the case of a three-phase machine; with a single-phase machine one of the lines, say 
CC, is omitted. 


Machine 





Fira. 1.—Synchronizing Lamps Connected 
Across the Motor Switch. 


Copyright, 1913. Published by Joun Witry & Sons, Ine, 


Some prefer to have synchronizing lamps crossed, as shown in Fig. 2. The machine is in 
synchronism when the lamps glow the brightest. As an advantage of this arrangement, it is 
claimed that a lamp should burn out during the process of synchronizing, the operator would 

immediately notice it, while with the first arrangement 
4 F he may judge, by the lamps being extinguished, that 
1 1 = vi ts 5 
ike go the machine is in perfect synchronism. Thus, he may 
¥ e —B, close the switch while the machine is altogether out of 
phase, and, unless the protective devices (fuses or circuit 
breakers) operate promptly, the machine may be dam- 
aged by the rush of current. However, the possi- 
bility of a synchronizing lamp burning out is rather remote, and, with two- or three-phase 
machines, the burning out of one set of lamps would not affect the others. On the other hand, 
itis generally considered that it is easier to observe moments of total extinguishing than 
moments of maximum brilliancy. 

With three-phase machines, crossing synchronizing lamps in two phases is very convenient, 
especially when the lamps are arranged in a circle, as in Fig. 3. In this case, maximum bright- 
ness occurs in the three sets of the lamps in rotation, so 
that the light appears traveling along the circle. The 
direction in which the light rotates depends on whether the 
speed of the machine is low, or high. When the machine 
is in synchronism, the lamps marked “3” are dark, while 
the lamps “‘ 2” and “1” glow brightly. 

If a 220-volt single-phase motor is to be synchronized, 
at least four 110-volt synchronizing lamps should be used 
in series, because at certain moments during synchronizing 
the e.m.fs. of the machines may be acting in the same direc- Fic. 3.—Synchronizing Lamps Crossed in 
tion instead of in opposition, thus giving 440 volts. It is Two Phases. 
even better to have 5 lamps in series, so as not to let them 
glow too brightly; it is easier to observe the periods of the extinguishing of the light. With 
two three-phase machines of the same voltage, the pressure across the lamps in each phase can 
never exceed 220 volts, so that two 110-volt lamps in series are sufficient, though three lamps may 
give a better service. 

In synchronizing three-phase machines, lamps must always be provided in at least two 
phases. It is not sufficient to have the lamps in one phase only, because when phase A is 
connected to Ai phase, B may be connected to C; and C to Bj, thus causing a partial 
short-circuit. 

Ordinary voltmeters may be used for synchronizing, instead of lamps; a voltmeter can be 
safely connected between the machines because of its high resistance. When the machines are 
in synchronism, the voltmeter pointer comes to zero; otherwise it swings to and fro. 

During the last few years, synchronizing lamps fae gradually given place to special cynelnonine 
ing instruments, so-called synchroscopes, or synchronism indicators. These devices have 
the appearance of ordinary switchboard instruments, except that the pointer has no retaining 
spring or weight, and is free to revolve through 360 degrees. When the speed of the alternator 
to be synchronized is low, the pointer revolves in. one direction; if it is high the pointer 
rotates in the opposite direction. When the specd is right, the pointer stands still; and when 
the machine is ‘in phase,” the pointer shows zero, indicating that the main switch may be 
closed.* 

Connections.—The armature of the motor is connected to the line with synchronizing lamps 
placed across the switch. In addition to this switch, it is advisable to have an overload circuit 
breaker. which would protect the motor should the synchronizing switch be closed at a wrong 
instant. Have an ammeter in one of the phases and a voltmeter across one of the phases of the 
machine. The line voltage can be measured once for all before the beginning of the experiment. 
The field winding of the motor is connected to a source of direct current, in series with an ammeter, 
a rheostat and a switch. 





Fic. 2—Synchronizing Lamps Crossed. 





*The same methods are used in synchronizing alternators and rotary converters. 


Order of the Experiment.—1. Knowing the frequency of the supply and the number of 
poles of the motor calculate its required speed. For instance, if the frequency of the supply is 
50 cycles per second, and the number of poles is 6, we reason as follows: There are two alter- 
nations or one cycle per pair of poles; hence, three cycles per one revolution of the machine. 
The frequency is 50X60=3000 cycles per minute, consequently the required number of revolu- 
tions per minute is 3000/3=1000. Bring the motor to this speed by means of the available 
drive. 

2. Excite the field so that the induced voltage is nearly equal to the line voltage. The lamps 
are now flickering. 

3. Regulate the speed of the motor until the lamps show almost a perfect synchronism, 
that is they light up and grow dim slowly. Close the switch at the right moment; do not bein 
a hurry, but act decisively when you are ready. Disconnect the drive from its source of power, 
or throw off the belt to convince yourself that the motor is running from the alternating-current 
supply. Apply some mechanical pressure to its pulley in order to see that the motor is capable 
of carrying a load. 

4. Try different methods of synchronizing, that is with the lamps dim, bright, and the 
light revolving, according to Figs. 1, 2, and 3. 

5. Try synchronizing with a voltmeter, and with a synchronism indicator, if one is 
available. 

6. Decide upon the best conditions for synchronizing and then synchronize the motor several 
times in succession, starting every time with the motor at rest and with no field. Make a note 
of the least number of seconds during which you succeeded in synchronizing the machine and 
making it ready for the load. Promptness in synchronizing is of great practical consequence when 
a synchronous machine carries an important load. 

7. See what effect is produced if the motor is switched in without having some one of the 
three above-mentioned conditions fulfilled. This produces a partial short-circuit; therefore be 
sure that the machine is protected by a reliable circuit breaker. 

8. After the motor has been synchronized, increase the field current above the normal, and 
then reduce it below the normal. You will find that in either case the current taken from the line 
is increased. Since the losses in the machine are practically the same, the additional current 
must be wattless (reactive). Theory and experiment show that, when a synchronous motor is 
over-excited, it draws a leading current from the line. This relation is often used in order to 
improve the power factor of the load. An under-excited synchronous motor draws a lagging cur- 
rent from the line, which is usually undesirable. If a power-factor meter is available, the existence 
cf the lagging or leading currents, depending upon the excitation, may be shown directly. If 
not, connect reactance coils between the switch and the motor, one in each phase; adjust the react- 
ances so that 10 to 20 per cent of the line voltage is consumed in them. Measure the motor voltage 
with the field under-excited and over-excited. It will be found that in the first case the motor 
voltage is below that of the line, in the second case it is higher than the line voltage. But, from 
the general theory of alternating currents, it is known that a leading current through a reactance 
causes a negative drop, in other words the line voltage is increased. 

9. If the drive is a direct-current, shunt-wound motor, an interchange of power between the 
two sources of power supply may be arranged. Having synchronized the alternating-current 
motor, raise the excitation of the direct-current motor. Its counter-e.m.f. becomes larger than 
that of the line and it begins to act as a generator, “pumping” power into the direct-current 
line at the expense of the power delivered from the alternating-current line. The direct- 
current line ammeter shows a reversed current. Now weaken the field of the direct-current 
machine below normal. In its tendency to run faster it drives the alternating-current motor 
so that the latter begins to “pump” power into the alternating-current line, acting as an 
alternator. Having an indicating wattmeter in the alternating-current line, this reversal in 
the sign of power can be observed directly. Having measured the watts output of one 
machine and the input into the other machine, the efficiency of the set can be calculated. 
Take such readings with either machine working as a generator. 

Report.—1. Give the exact diagrams of the connections used. 

2. Give detailed instructions for synchronizing under the conditions which you have 


found to be the best, and state how many seconds it takes to synchronize under these con- 
ditions. 

3. Describe what happened when the machine was switched in without being brought to 
exact synchronism. 

4. State how you proved the existence of lagging and leading currents in the armature, and 
give theoretical reasons for their existence. 

5. Show how to calculate the efficiency of the set from the input and the output, and give 
your data and results. 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 212-1. ASSEMBLING AND OPERATING A DIRECT-CURRENT SWITCH- 


BOARD 
Apparatus.—As per Fig. 2, or Fig. 3. 


The purpose of the experiment is to become acquainted with the arrangement of apparatus 


Field 
Windings 






Fuses or 
Circuit_Breaker 


Field Rheostat 


Fic. 1.—Connections for the Generator. 









on small and medium-sized switchboards used in connection with direct-current generators, for 


instance in isolated power plants. 


One Generator.—With one generator (Fig. 2) the connections are practically the same as 
in Fig. 1, already familiar to the student from the experiments on characteristics of direct-current 


generators, except that in practice the field ammeter and the field 
switch are usually omitted. The ammeter and the voltmeter are 
mounted near the top of the switchboard; between them is visible 
the handle of the field rheostat. The large switch in the center 
connects the machine to the switchboard; the four smaller switches 
are for the outgoing feeders. All the connections are made on the 
back of the switchboard. The little circles shown on each side of 
the instruments denote ground-detector lamps. Each lamp is con- 
nected between one terminal of the machine and the ground (for 
instance a water pipe). As long as the insulation of the machine 
is good, the lamps are dark, but when one side becomes grounded, 
the lamp on the other side lights up. Each switch circuit is pro- 
tected by fuses, visible under the switches. Automatic circuit 
breakers are coming more and more into use instead of switches 
and fuses. The main switch is connected on the back to two 
horizontal copper bars, commonly called bus-bars, so that the gen- 
erator power is delivered to the bus-bars. The feeder switches are 
also connected to the bus-bars, and in this way the energy taken 
from the generator is delivered to various feeder circuits. 

Two Generators.—Switchboard connections for two compound- 
wound generators are shown in Fig. 3. The two outside panels 
are generator panels. The middle panel is for the outgoing feeders. 
The left-hand panel is shown with all the connections; the right- 
hand panel is left unconnected. The main bus-bars extend 
throughout the whole length of the switchboard. The negative 














Fig. 2.—Switchboard for a 
Single Generator. 


terminals of the machines are connected directly to the negative bus-bar, through the main 
switches. The positive cables are connected to the corresponding bus-bar, through the circuit 
breakers and the ammeters. One terminal of each field circuit is taken to the switchboard, 


Copyright, 1913, by V. Karaprrtorr, Published by Joun Wiuuy & Sons, Inc, 


in order to have it connected to the field rheostat. One voltmeter is used for both machines. 
It may be connected to either machine by means of a receptacle and a plug. Each generator panel 
is provided with a lamp which serves for illuminating the ammeter scale and also as a pilot lamp. 





O Pilot Lamp 


Oo Ammeter 
Circuit 


Y Ammeter 
Shunt 
a | 


























Feeder Panel 5 Generator Panel No.2 





Series Field 





Shunt Field 


Fic. 3.—Switchboard Connections for Two Compound-wound 
Generators. 


The feeder panel has three lamps on top. The middle one is connected across the bus-bars 
and illuminates the voltmeter scale; the two outside ones are ground-detector lamps. Five 
feeder switches are shown on the middle panel, each circuit being protected by fuses. Circuit 

breakers, taking the place of both fuses and switches, are much used at present. 
The equalizing connection shown between the positive brushes of the machines is used with 
compound-wound generators only, its purpose 
Jc aik picemae Ree Bus Bars : being to make the two machines divide the 

a 2 
ay ag 

{ Gi 000 higher and therefore the current supplied by 
it larger than that of machine No. 1. This 
tends to make the ohmic drop between a2 and 
thus part of the current of machine No. 2, instead of flowing from ag to the positive bus-bar 
directly through bz, flows to the same bus-bar through the equalizer c and the series winding of 
machine No. 1. Therefore, the field of machine No. 2 is strengthened less than it would 


load equally. The action of the equalizer is 

Series Field 
Fia. 4.—Equalizing Connection for Compound-wound 2 larger than that between a; and b6,; but, 
be without the equalizer; at the same time the field of the weaker machine, No. 1, is strength- 






as follows: suppose (Fig. 4) that for some 
reason the induced e.m.f. of machine No. 2 is 
Generators. with an equalizing cable of a negligible resist- 
ance between a and ag, this is impossible, and 


od 


ened by the excess of the current of the other machine; machine No. 1 is thus helped to keep 
up its voltage. In short, the equalizing connection prevents the currents in the series fields of 
two or more machines from differing widely from each other, however different their armature currents 
may be. Therefore, when connected by an equalizing bus-bar, different machines cannot have 
widely different voltages, cannot easily have the load disproportionately distributed, and the 
more remote becomes the possibility of one machine pumping power back into the other machine. 

The Wiring.—Much of the benefit derived from this experiment depends upon a neat wiring 
on the back of the switchboard. For experimental purposes the switchboard itself may consist 
simply of a few wooden boards, with holes drilled to receive the studs of the instruments and the 
switches. First, place two bus-bars on the back, supporting them from simple brackets properly 
insulated. Then wire up the main circuit, and finally the voltmeter connections, ground detectors, 
etc. Then connect the machine (or machines) to the switchboard. 

Operating the Switchboard.—Imagine yourself to be a switchboard operator in an isolated 
power plant, and perform the operations which he would have to perform under normal conditions. 

1. In a plant with one generator only, start the machine, excite it properly, close the main 
switch, and finally load it. Then perform the operations necessary for shutting down the plant. 

2. Operate the plant with two shunt-wound machines in parallel, transferring the load at will 
from one machine to the other by regulating the field rheostats. Start for instance with machine 
No. 1, have it loaded, then connect No. 2 in parallel, divide the load equally, transfer the load 
to No. 2, and disconnect No. 1. 

3. If conditions permit, operate two compound-wound machines in parallel, with and with- 
out an equalizing connection, so as to see the purpose of such a connection. 

Report.—1. Draw a neat diagram of the actual connections if different from those shown 
in Figs. 2 and 3. 

2. Write explicit and concise directions as to the order in which operations must be per- 
formed when starting the plant, paralleling the machines, changing the load from one machine 
to the other, and shutting down. 

3. Show how to calculate the size (rating in amperes) of the switches,ammeters, and conductors 
in a plant of a given size. Assume the permissible overload to be 25 per cent. 





THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 213-1. TEST OF A LIFTING MAGNET 


Apparatus.—Suitable electromagnet; iron or steel armature for the same; ammeter; regu- 
lating rheostat; weights. 

The purpose of the experiment is to determine the relation between the current consumed 
by an electromagnet, under different conditions of service, and its lifting power. The theoretical 
formula for the mechanical force, /’, of attraction between the core and the armature is 

B?A 


jpeg 8 
7213.” 


where B is the magnetic flux density in the air-gap, in kilolines per square inch, and A is the 
total area of contact between the armature and the iron core, in square inches. This includes the 
inner area and the concentric outer area. In the metric system 


where B is in kilolines per square centimeter, and A is the area in square centimeters. It will 
be seen from these expressions that the lifting power increases as the square of the flux density 
in the air-gap. Without saturation in iron, the flux density would be proportional to the excit- 
ing current, and the lifting power would be proportional to the square of the current. However, 
as saturation sets in, the lifting power increases more slowly than the square of the current. 

The flux density, at a certain current, depends upon the reluctance (magnetic resistance) 
of the paths of the lines of force. Consequently, by inter- 
posing a sheet of paper or fiber between the core and the 
armature, the lifting power is considerably reduced. The 
same effect is produced when the surface of the object to be 
lifted is irregular and touches the core only in a few points, 
Again, by substituting a cast-iron armature in place of a 
steel one, the reluctance of the magnetic circuit is increased 
and the lifting power of the magnet is reduced. The mag- 
net may be loaded by suspending weights directly on the 
hook of the armature, but it is sometimes more convenient 
to introduce a leverage. The magnet is made to pull 
upward on the short arm of a lever, and a comparatively: 
small weight is suspended at the end of the long arm. In 
this way one need not handle large weights. The pull is to 
the weight as the inverse ratio of the arms. 

Data Sheet.—Record exciting amperes, and the weights 
which are necessary in order to pull the armature from 
the core. The weight of the armature must, of course, be ae 
included. Lifting Magnet. 

Readings.—1. Excite the armature with the highest 
available current, or that which is safe for the winding. Load the armature until it drops. 
Note the current and the weight used. Reduce the current, and repeat the test, etc. From 
eight to ten points are necessary for a good curve. Having reduced the current, do not increase 
it again; otherwise you will be following a different hysteresis loop (see experiment E 201-1). 
Having reduced the current to zero, repeat the test with an increasing current so as to see the 
influence of the residual magnetism. 


ta 





Copyright, 1913, by V. Karaprtorr, Published by Joun Witty & Sons, Inc, (OVER) 


2. Repeat the test, using a definite thickness of paper or fiber between the armature and the 
core. 

3. Repeat the test using a different armature, for example one made of cast iron if the first 
one was of cast steel. Before leaving the laboratory weigh the armature used. 

Report.—1. Plot the lifting power in pounds or in kilograms to amperes as abscissee. Use 
the same curve sheet and the same scale for all the curves so as to make a direct comparison 
possible. 

2. Indicate by dotted lines one or two theoretical curves which would obtain if the lifting 
power continued to increase indefinitely as the square of the current (parabola). 

3. For a certain excitation which is assumed to be normal or rated for the magnet, tabulate 
the corresponding flux densities with the two armatures used, and when a layer of non-magnetic 
material was interposed; use the formula given above, solving it for B. 

4, Answer the following questions: 

(a) Why are lifting magnets made with a closed magnetic circuit resembling a horse- 
shoe magnet rather than a bar magnet? 

(b) To obtain a given lifting power with a given core, a certain number of ampere- 
turns is necessary. What determines the number of turns and the current in 
practice, seeing that only the product of these two quantities is given? 

(c) In what cases would you use a lifting magnet in preference to an ordinary hook, 

in connection with a traveling crane? 


THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 213-1. TEST OF A LIFTING MAGNET 


Apparatus.—Suitable electromagnet; iron or steel armature for the same; ammeter; regu- 
lating rheostat; weights. 

The purpose of the experiment is to determine the relation between the current consumed 
by an electromagnet, under different conditions of service, and its lifting power. The theoretical 
formula for the mechanical force, F’, of attraction between the core and the armature is 


where B is the magnetic flux density in the air-gap, in kilolines per square inch, and A is the 
total area of contact between the armature and the iron core, in square inches. This includes the 
inner area and the concentric outer area. In the metric system. 


where B is in kilolines per square centimeter, and A is the area in square centimeters. It will 
be seen from these expressions that the lifting power increases as the square of the flux density 
in the air-gap. Without saturation in iron, the flux density would be proportional to the excit- 
ing current, and the lifting power would be proportional to the square of the current. However, 
as saturation sets in, the lifting power increases more slowly than the square of the current. 

The flux density, at a certain current, depends upon the reluctance (magnetic resistance) 
of the paths of the lines of force. Consequently, by inter- 
posing a sheet of paper or fiber between the core and the 
armature, the lifting power is considerably reduced. The 
same effect is produced when the surface of the object to be 
lifted is irregular and touches the core only in a few points. 
Again, by substituting a cast-iron armature in place of a 
steel one, the reluctance of the magnetic circuit is increased 
and the lifting power of the magnet is reduced. The mag- 
net may be loaded by suspending weights directly on the 
hook of the armature, but it is sometimes more convenient 
to introduce a leverage. The magnet is made to pull 
upward on the short arm of a lever, and a comparatively 
small weight is suspended at the end of the long arm. In 
this way one need not handle large weights. The pull is to 
the weight as the inverse ratio of the arms. 

Data Sheet.—Record exciting amperes, and the weights 
which are necessary in order to pull the armature from 
the core. The weight of the armature must, of course, be oe 
included. Lifting Magnet. 

Readings.—1. Excite the armature with the highest 
available current, or that which is safe for the winding. Load the armature until it drops. 
Note the current and the weight used. Reduce the current, and repeat the test, etc. From 
eight to ten points are necessary for a good curve. Having reduced the current, do not increase 
it again; otherwise you will be following a different hysteresis loop (see experiment E 201-1). 
Having reduced the current to zero, repeat the test with an increasing current so as to see the 
influence of the residual magnetism. 


+ 





Copyright, 1913, by V. KarapETorr, Published by Joun Witey & Sons, Inc. (OVER) 


2. Repeat the test, using a definite thickness of paper or fiber between the armature and the 
core. 

3. Repeat the test using a different armature, for example one made of cast iron if the first 
one was of cast steel. Before leaving the laboratory weigh the armature used. 

Report.—1l. Plot the lifting power in pounds or in kilograms to amperes as abscisse. Use 
the same curve sheet and the same scale for all the curves so as to make a direct comparison 
possible. 

2. Indicate by dotted lines one or two theoretical curves which would obtain if the lifting 
power continued to increase indefinitely as the square of the current (parabola). 

3. For a certain excitation which is assumed to be normal or rated for the magnet, tabulate 
the corresponding flux densities with the two armatures used, and when a layer of non-magnetic 
material was interposed; use the formula given above, solving it for B. 

4. Answer the following questions: 

(a) Why are lifting magnets made with a closed magnetic circuit resembling a horse- 
shoe magnet rather than a bar magnet? 

(b) To obtain a given lifting power with a given core, a certain number of ampere- 
turns is necessary. What determines the number of turns and the current in 
practice, seeing that only the product of these two quantities is given? 

(c) In what cases would you use a lifting magnet in preference to an ordinary hook, 

in connection with a traveling crane? 


THE LOOSE LEAF LABORATORY MANUAL 
ELECTRICAL TESTING 


EXPERIMENT E 214-1. OPERATING MOTOR STARTERS WITH NO-VOLTAGE AND 
OVERLOAD RELEASE © 


Apparatus.—One or more motor-starters and speed regulators with automatic no-voltage 
and overload features; shunt-wound motor; ammeter; voltmeter; switch and fuses. 

The purpose of the experiment is to acquaint the student with the principal types of start- 
ing and regulating devices used in practice in connection with direct-current motors. Small 
motors are usually operated and taken care of by persons of limited electrical training, and the 
rheostats must be designed to meet the severe conditions of usage. It is impossible in a limited 
space to give a description of various types of starting and regulating rheostats with automatic 
protective features used in practice. With a little experience the construction and the functions 
of a given device are easily ascertained. To assist the student, his attention is called to the 

principal requirements in the operation of 
shunt-wound motors; these requirements 
will be found incorporated in motor starters 
and regulators with which he will be called 
upon to deal. 

(1) No-voltage Release (Fig. 1).—One 
fundamental requirement which every motor 
starter must satisfy is that its whole resistance 
must be automatically introduced in series with 

. the motor armature as soon as the main- 
switch is opened or the power is “ off” for 
any reason whatsoever. Otherwise, when 
the power is ‘‘ on ” again, or the main switch 

(st is carelessly closed, a current would flow 

through the armature equal to many times 

Fic. 1.—Motor Starter with No-voltage Release. its rated current. The result would be that 

either the fuse would blow out, or the circuit 
breaker would not stay “in’’; it is also possible that damage would be done to the motor. 

Therefore, the operating handle of a motor starter is held in its running position by the attrac- 

tion of an electromagnet energized from the line. Should the 
power be “‘ off,” the electromagnet is de-energized, the handle 
is released, and it flies back to its starting position under the 
action of a spring. The coil of the electromagnet is con- 
nected to the line either directly or in series with the shunt field 
winding of the motor. In the latter case it protects the motor 
if the field circuit is broken, even though the power is still on. 

(2) Overload Release.—A motor circuit can be protected 
by a circuit breaker or fuses like any other circuit. But when 
the conditions of the service are such that the motor is 
frequently overloaded, fuses are unsuitable, being expensive 
and causing delays. A circuit breaker can be incorporated 
into one apparatus with the motor starter. The advantages 
of this arrangement are that the combined apparatus costs 
less, and the attendant has only one handle to operate, so 
that he cannot perform the operations in a wrong order. ; 
There are two principal types of motor starters with an over- Fie. 2.—Motor Starter with No-volt- 

; ‘ age and Overload Release. 
load feature. In the simplest type a small electromagnet is 
provided, the coil of which is connected in series with the line. When the current through the 


Copyright, 1913, by V. KarapeTorr, Published by JoHn Witey & Sons,. Inc. 








motor exceeds a certain limit, this electromagnet attracts its armature. The latter strikes a contact 
which short-circuits the coil of the no-voltage release magnet described under (1) above. The 
starting handle is released and the motor stopped. In the other type (Fig. 2) the overload 
release electromagnet c holds a separate blade, k, which, on an overload, is released by the latch ¢, 
flies off and forms one lever with the starting arm. To start the motor again, it is necessary to 
return the starting lever to its starting position, because in this position the blade k closes the 
main circuit at 7 and is held by the overload electromagnet. The knurled head r is for the pur- 
pose of adjusting the overload electromagnet to trip at a desired value of the current. The 
second type is more positive in its action, but is somewhat more expensive. 

(3) Field Control—In the above-described starting rheostats, no provision is made for 
regulating the field current of the motor, in other words, for varying its speed. If speed control 
is required, an additional rheostat must be connected into the field circuit. But it must be remem- 
bered that the motor should always be started with the strongest field, in order to get a good 
starting torque without an excessive rush of current. Therefore, the starting and field rheostats 
must be suitably interlocked, either mechanically or electrically. 

A device of this kind is shown in Fig. 3. The lower row of contacts is connected to the 
starting resistance, the upper row to the field rheostat. A double lever is provided, the 
outside arm being for the field contacts, the 
inside one for the starting contacts (see side 
view to the right). The outside arm only is 
provided with an operating handle. In start- 
ing, the two arms are moved together, but 
the field arm is electrically inoperative, be- 
cause the field current flows directly through 

A |Z the starting-lever, the bar 6, the solenoid, 
Starting-|{]] and into the field of the motor. At the end 
Boy A’ of the starting period, the starting-lever is 
attracted and held by the no-voltage release 
coil, while the field lever may be moved 
back to increase the speed of the motor. The 
upper row of contacts is now operative, since 
the starting-lever no longer touches the 


: short-circuiting bar b, but rests on the blind 
Fie. 3.—Combined Motor, Starter and Speed Regulator. yytton d. 





Spring 





Opening the main switch releases the 
starting-lever, which flies back, strikes on its way the field lever, and both levers are returned 
to the “zero” position. It will be seen from the above description, that it is impossible to 
start the motor with a weakened field. An overload feature, similar to that in Fig. 2, may be 
added to this starter. 

Order of the Experiment.—(a) Starter with a no-voltage release. 

1. Wire up the motor and the starting-box, and practise starting and stopping. Make clear 
to yourself the order in which the main switch and the handle of the rheostat must be operated, 
and to what extent the arrangement is “ fool-proof;’ also what would happen if the operations 
are performed in a wrong order. 

2. Explain why the handle does not fly back immediately after the main switch is opened; 
prove the explanation by an experiment. Determine the minimum line voltage at which the coil 
can hold the arm. Interpose pieces of thin paper between the coil and its armature, and observe 
the effect on the magnetic attraction. 

3. Apply a certain brake load and start the motor by moving the rheostat arm at a certain - 
definite speed, using a metronome. Read the instantaneous values of the line amperes and 
the volts across the armature every few seconds. With three observers, after some practice, 
it is possible to take readings on an instrument every two seconds. One man signals at the proper 
time, another reads aloud the scale indications, the third records the readings. Repeat the 
same experiment with different rates of starting, and with different values of the load. 

4. Measure the total resistance of the starting-box, and the resistances of the separate steps. 


This is done by putting a steady current through the rheostat, and taking the voltage drop 
between adjacent buttons. 

(b) Starter with an overload release—Two types of starters may be investigated as described 
above. It is well to test, in addition, an ordinary starter with no-voltage release (Fig. 1), using 
a separate overload circuit breaker. 

1. Connect the devices in succession to a motor, and practise starting. Observe the action 
of the overload protection. Make clear to yourself that no wrong move is possible, except with 
deliberate intention. 

2. Calibrate the overload attachment in amperes. For this work use an ordinary load rheostat 
instead of a motor; the main current can be kept more constant. 

3. Make tests of the influence of the “ time element ” on the action of the overload attachment. 
Adjust a certain current through the overload coil, and then increase the current by a certain per 
cent, first gradually, then instantaneously, and observe the difference in the operation of the trip- 
ping mechanism. Perform this experiment with different values of current, and with different 
percentages of increase. 

4. Compare the action of fuses and of a circuit breaker on slow and sudden overload; obtain, 
if possible, definite numerical results. 

(c) Speed regulator.—1. Connect the speed regulator to a motor and practise operating it; 
make clear to yourself the automatic features of the device. 

2. Measure the speed and field current of the motor with several positions of the regulating 
handle. 

3. Measure the total resistance of the field rheostat, and the resistances of the separate steps, 
also the resistance of the motor field. 

4. Connect an ordinary motor starter (Fig. 1) and a separate field rheostat in place of the 
combination starter and regulator. Devise an electrical or a mechanical interlocking arrange- 
ment, which would prevent starting the motor with a weakened field. 

Report.—1. Make neat sketches indicating the mechanical features and the electrical con- 
nections of the devices investigated. 

2. Write explicit directions for the operator, and a few warnings as to what not to do. 

3. Give the readings of volts, amperes, ohms, speed, seconds, etc., taken during the experi- 
ment, and draw your conclusions therefrom. 

4, Give your criticisms, favorable or otherwise, of the devices investigated, suggest improve- 
ments if any, and some different arrangements which would go around the patents. 


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THE LOOSE LEAF LABORATORY MANUAL 


ELECTRICAL TESTING 


EXPERIMENT E 215-1. WIRING A MACHINE TOOL CONTROLLER 


Apparatus.—Experimental, drum-type controller; variable-speed shunt motor; starting 
rheostat; field rheostat; main ammeter: field ammeter; voltmeter; switch and fuses (or a 
circuit breaker). 

The purpose of the experiment is to familiarize the student with the construction and opera- 
tion of a machine-tool controller, used in connection with a shunt-wound motor. A machine 
tool, such as a lathe, if driven by an individual electric motor, usually must be operated within 
quite a wide range of speeds without changing the gears, and sometimes must be reversed. The 
motor must be provided with a variable resistance for starting, and also with a field rheostat, if 
speed adjustment is required; besides, there must be a switch in the main circuit. If it is nec- 
cessary to operate the motor in both directions, a double-throw switch must be added, so connected 
that it reverses the current either in the armature alone or in the field only. Sometimes motors 
are operated on a three-wire system, in which case the connections become still more complicated, 
especially if the motor must be reversible. 

But the use of three or more separate switches and regulating devices cannot be tolerated 
in practice, this being too awkward and complicated for the operator. It is particularly objec- 
tionable in cases where motors are started and reversed many times a day, or are intrusted to 
persons incompetent in electrical matters, for instance to machine-tool operators. All, or practi- 
cally all, the necessary switches and rheostats must either be combined into one device, or must be 
mutually interlocked, so as to make operation in a wrong order impossible. Such a combination 
apparatus is called a controller. 

Controller.—The most common form of controller is the drum-type similar to the familiar 
street-car controller. ‘The wires coming from the line, from the motor and from the starting and 
regulating resistances, are all connected to stationary controller “ fingers,’ and these are brought 
into the necessary combinations by the connecting copper pieces, mounted on the revolving drum. 
The drum is operated by a handle, and in each position of the handle various fingers are con- 
nected in a different way, so as to vary the speed of the motor, the direction of rotation, etc. 

An ordinary machine-tool controller has, in the most general case, the following three duties 
to perform: to start the motor, to reverse the motor, and to vary the speed. However com- 
plicated the connections inside the controller may be, the machinist does not need to know about 
them; all he has to do is to turn the handle one 


way or the other, the controller does the rest. = 3 aE 

The elementary controller connections are shown 
in Figs. 1 to 4. In all these diagrams the control- re shee eae 
ler drum is shown developed on a plane; different ‘Shunt Field at oy 
positions of the fingers a, b, c, etc., on the copper Ss a 2 
strips x, y, 2, are indicated by dotted vertical lines. Retaneics 











(a) Starting connections (Fig. 1).—On the first | 


notch the current from one terminal of the line saree Be 
passes through the finger a, the strips 21, 74, x3, %2, — 
and 2; to the finger e, thence through the whole Resistance = od 


starting resistance to the armature of the motor and a 
out to the other terminal of the line. On the 
second notch the finger d touches the strip 2, 
and part of the starting resistance, that between Fig. 1.—Starting Connections. 
d and e, is cut out. On the third notch still more 
resistance is cut out, and finally, on the fourth notch, the current flows through a, z, x4 and b 
direct to the armature, without any starting resistance in series; this is the running position of 
the drum. 

Copyright, 1913, by V. Karaperorr. Published by JoHN Wiuey & Sons, Inc, 


(b) Speed control. by means of a variable resistance in the field circuit (Fig. 2)—For the sake 
of clearness the starting connections are omitted. On the fifth notch the field is excited directly 
across the line, without any resistance in series with it. This gives the strongest field, and 
therefore the lowest speed. On the sixth notch the resistance between the fingers g and h is 
inserted into the circuit, on the next notch that between g and 7, etc., until on the last notch the 
whole field resistance is put into the circuit, and the motor runs at its highest speed. 

(c) Reversing the motor (Fig. 3)—When the drum is in the “ Forward ”’ position, the current 


Armature 


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Controller Positions 


Shunt Field 


Field ~ 
Resistance 








Fig. 2.—Resistance in the Field Circuit. Fig. 3.—Reversing the Motor, 


from the positive terminal flows through n, w1, w2, and m to the armature terminal Aj, and thence 
returns to the line through the terminal Ay. When the controller handle is in the “ Reverse ” 
position, the current passes through n, v1, v3, and / to the armature terminal Ag, thus flowing through 
the armature in the opposite direction. Therefore, the motor now runs in the opposite direction, 
the field connections not being reversed. 

(d) Speed control by means of a three-wire supply (Fig. 4)—Assume, for example, that the supply 
gives 125 volts and 250 volts. In the position marked ‘ Half- 
speed ” the armature is connected between the positive and the 
neutral (--) wires; at full speed it is connected between the 
positive and the negative terminals. 

Experimental Controller—When studying the connections 
and experimenting with an actual controller, the student is 
handicapped by the fact that the controller is all wired up, and 
some of the wiring is not accessible. Moreover, the controller 
is usually intended for a specific duty only, and cannot very 

Fic. 4—Three-wire Supply. well be used for various purposes. 

It is therefore advisable to have in the laboratory an experi- 
mental controller, especially adapted for exercises in wiring. No permanent connections should be 
made between the strips on the drum, but each strip should be provided with one or more binding 
posts so that the student may establish any desired connections himself. Some of the strips 
must be long, others short, and arranged stepwise, for gradually cutting in or out resistances. 
Such a controller, if properly designed, is very useful for a study of the operations explained 
above. 

The controller should be mounted horizontally in order to be more accessible, and should have 
no cover, save that there must be a board on which the fingers are mounted. It is not advisable 
to have a blow-out coil in connection with it, in order to keep the device as simple as possible. 
The student should be given an opportunity to study the action of a magnetic blow-out on a sep- 
arate electromagnet. 

Order of the Experiment.—1. Connect up the controller for starting a shunt motor in one 
direction only, without field control. 





2. Add the connections necessary for field control. 

3. Supplement the connections by those required for reversing the motor. 

4. Wire up the controller complete for running forward and reverse, on a three-wire system. 

A shunt motor should be provided, and operated in connection with the controller, this is the 
best check on the connections. Have an ammeter in the armature circuit, and one in the field 
circuit; also a voltmeter across the armature terminals. Measure the speed of the motor with 
various positions of the controller handle. 

At the end of the experiment remove all the connections in the controller, so that the next 
students may have the benefit of designing their own connections. 

Report.—Draw a diagram of the actual connections used, combining the developments 
shown in Figs. 1 to 4. Give the numerical data in regard to the performance of the motor on 
the different notches. 














